WASHINGTON — The Pentagon will send hundreds of additional spies overseas as part of an ambitious plan to assemble an espionage network that rivals the CIA in size, U.S. officials said.

The project is aimed at transforming the Defense Intelligence Agency, which has been dominated for the past decade by the demands of two wars, into a spy service focused on emerging threats and more closely aligned with the CIA and elite military commando units.

When the expansion is complete, the DIA is expected to have as many as 1,600 “collectors” in positions around the world, an unprecedented total for an agency whose presence abroad numbered in the triple digits in recent years.

The total includes military attachés and others who do not work undercover. But U.S. officials said the growth will be driven over a five-year period by the deployment of a new generation of clandestine operatives. They will be trained by the CIA and often work with the U.S. Joint Special Operations Command, but they will get their spying assignments from the Department of Defense.

Among the Pentagon’s top intelligence priorities, officials said, are Islamist militant groups in Africa, weapons transfers by North Korea and Iran, and military modernization underway in China.

“This is not a marginal adjustment for DIA,” the agency’s director, Lt. Gen. Michael Flynn, said at a recent conference, during which he outlined the changes but did not describe them in detail. “This is a major adjustment for national security.”

The sharp increase in DIA undercover operatives is part of a far-reaching trend: a convergence of the military and intelligence agencies that has blurred their once-distinct missions, capabilities and even their leadership ranks.

Through its drone program, the CIA now accounts for a majority of lethal U.S. operations outside the Afghan war zone. At the same time, the Pentagon’s plan to create what it calls the Defense Clandestine Service, or DCS, reflects the military’s latest and largest foray into secret intelligence work.

The DIA overhaul — combined with the growth of the CIA since the Sept. 11, 2001, terrorist attacks — will create a spy network of unprecedented size. The plan reflects the Obama administration’s affinity for espionage and covert action over conventional force. It also fits in with the administration’s efforts to codify its counterterrorism policies for a sustained conflict and assemble the pieces abroad necessary to carry it out.

Unlike the CIA, the Pentagon’s spy agency is not authorized to conduct covert operations that go beyond intelligence gathering, such as drone strikes, political sabotage or arming militants.

But DIA has long played a major role in assessing and identifying targets for U.S. forces, which in recent years have assembled a constellation of drone bases stretching from Afghanistan to East Africa.

The expansion of the agency’s clandestine role is likely to heighten concerns that it will be accompanied by an escalation in lethal strikes and other operations outside public view. Because of differences in legal authorities, the military isn’t subject to the same congressional notification requirements as the CIA, leading to potential oversight gaps.

U.S. officials said that DIA’s realignment won’t hamper congressional scrutiny. “We have to keep congressional staffs and members in the loop,” Flynn said in October, adding that he believes the changes will help the United States anticipate threats and avoid being drawn more directly into what he predicted will be an “era of persistent conflict.”

U.S. officials said the changes for DIA were enabled by a rare syncing of personalities and interests among top officials at the Pentagon and CIA, many of whom switched from one organization to the other to take their current jobs.

“The stars have been aligning on this for a while,” said a former senior U.S. military official involved in planning the DIA transformation. Like most others interviewed for this article, the former official spoke on the condition of anonymity because of the classified nature of the program.

The DIA project has been spearheaded by Michael Vickers, the top intelligence official at the Pentagon and a veteran of the CIA.

Agreements on coordination were approved by Defense Secretary Leon Panetta, a former CIA director, and retired Army Gen. David Petraeus, who resigned abruptly as CIA chief last month over an extramarital affair.

The Pentagon announced the DCS plan in April but details have been kept secret. Former senior Defense Department officials said that the DIA now has about 500 “case officers,” the term for clandestine Pentagon and CIA operatives, and that the number is expected to reach between 800 and 1,000 by 2018.

Pentagon and DIA officials declined to discuss specifics. A senior U.S. defense official said the changes will affect thousands of DIA employees, as analysts, logistics specialists and others are reassigned to support additional spies.

The plan still faces some hurdles, including the challenge of creating “cover” arrangements for hundreds of additional spies. U.S. embassies typically have a set number of slots for intelligence operatives posing as diplomats, most of which are taken by the CIA.

The project has also encountered opposition from policymakers on Capitol Hill, who see the terms of the new arrangement as overly generous to the CIA.

The DIA operatives “for the most part are going to be working for CIA station chiefs,” needing their approval to enter a particular country and clearance on which informants they intend to recruit, said a senior congressional official briefed on the plan. “If CIA needs more people working for them, they should be footing the bill.”

Pentagon officials said that sending more DIA operatives overseas will shore up intelligence on subjects that the CIA is not able or willing to pursue. “We are in a position to contribute to defense priorities that frankly CIA is not,” the senior Defense Department official said.

The project was triggered by a classified study by the director of national intelligence last year that concluded that key Pentagon intelligence priorities were falling into gaps created by the DIA’s heavy focus on battlefield issues and CIA’s extensive workload. U.S. officials said DIA needed to be repositioned as the wars in Iraq and Afghanistan give way to what many expect will be a period of sporadic conflicts and simmering threats requiring close-in intelligence work.

“It’s the nature of the world we’re in,” said the senior defense official, who is involved in overseeing the changes at the DIA. “We just see a long-term era of change before things settle.”

The CIA is increasingly overstretched. Obama administration officials have said they expect the agency’s drone campaign against al-Qaida to continue for at least a decade more, even as the agency faces pressure to stay abreast of issues including turmoil across the Middle East. Meanwhile, the CIA hasn’t met ambitious goals set by former president George W. Bush to expand its own clandestine service.

CIA officials including John D. Bennett, director of the National Clandestine Service, have backed DIA’s plan. It “amplifies the ability of both CIA and DIA to achieve the best results,” said CIA spokesman Preston Golson.

Defense officials stressed that the DIA has not been given any new authorities or permission to expand its total payroll. Instead, the new spy slots will be created by cutting or converting other positions across the DIA workforce, which has doubled in the past decade — largely through absorption of other military intelligence entities — to about 16,500.

Vickers has given the DIA an infusion of about $100 million to kick-start the program, officials said, but the agency’s total budget is expected to remain stagnant or decline amid mounting financial pressures across the government.

DIA’s overseas presence already includes hundreds of diplomatic posts — mainly defense attachés, who represent the military at U.S. embassies and openly gather information from foreign counterparts. Their roles won’t change, officials said. The attachés are part of the 1,600 target for the DIA, but such “overt” positions will represent a declining share amid the increase in undercover slots, officials said.

The senior Defense official said DIA has begun filling the first of the new posts.

For decades, the DIA has employed undercover operatives to gather secrets on foreign militaries and other targets. But the Defense Humint Service, as it was previously known, was often regarded as an inferior sibling to its civilian counterpart.

Previous efforts by the Pentagon to expand its intelligence role — particularly during Donald Rumsfeld’s time as defense secretary — led to intense turf skirmishes with the CIA.

Those frictions have been reduced, officials said, largely because the CIA sees advantages to the new arrangement, including assurances that its station chiefs overseas will be kept apprised of DIA missions and have authority to reject any that might conflict with CIA efforts. The CIA will also be able to turn over hundreds of Pentagon-driven assignments to newly arrived DIA operatives.

“The CIA doesn’t want to be looking for surface-to-air missiles in Libya” when it’s also under pressure to assess the opposition in Syria, said a former high-ranking U.S. military intelligence officer who worked closely with both spy services. Even in cases where their assignments overlap, the DIA is likely to be more focused than the CIA on military aspects – what U.S. commanders in Africa might ask about al-Qaida in Mali, for example, rather than the broader questions raised by the White House.

U.S. officials said DIA operatives, because of their military backgrounds, are often better equipped to recruit sources who can answer narrow military questions such as specifications of China’s fifth-generation fighter aircraft and its work on a nuclear aircraft carrier. “The CIA would like to give up that kind of work,” the former officer said.

The CIA has agreed to add new slots to its training classes at its facility in southern Virginia, known as the Farm, to make room for more military spies. The DIA has accounted for about 20 percent of each class in recent years, but that figure will grow.

The two agencies have also agreed to share resources overseas, including technical gear, logistics support, space in facilities and vehicles. The DIA has even adopted aspects of the CIA’s internal structure, creating a group called “Persia House,” for example, to pool resources on Iran.

The CIA’s influence extends across the DIA’s ranks. Flynn, who became director in July, is a three-star Army general who worked closely with the CIA in Afghanistan and Iraq. His deputy, David Shedd, spent the bulk of his career at the CIA, much of it overseas as a spy.

Several officials said the main DIA challenge will be finding ways to slip so many spies into position overseas with limited space in embassies. “There are some definite challenges from a cover perspective,” the senior defense official said.

Placing operatives in conventional military units means finding an excuse for them to stay behind when the unit rotates out before the end of the spy’s job.

Having DIA operatives pose as academics or business executives requires painstaking work to create those false identities, and it means they won’t be protected by diplomatic immunity if caught.

Flynn is seeking to reduce turnover in the DIA’s clandestine service by enabling military members to stay with the agency for multiple overseas tours rather than return to their units. But the DIA is increasingly hiring civilians to fill out its spy ranks.

The DIA has also forged a much tighter relationship with JSOC, the military’s elite and highly lethal commando force, which also carries out drone strikes in Yemen and other countries.

Key aspects of the DIA’s plan were developed by then-Director Ronald Burgess, a retired three-star general who had served as intelligence chief to JSOC.

The DIA played an extensive and largely hidden role in JSOC operations in Iraq and Afghanistan, sending analysts into war zones and turning a large chunk of its workforce and computer systems in Virginia into an analytic back office for JSOC.

The JSOC commander, Adm. William McRaven, who directed the operation that killed Osama bin Laden, has pledged to create between 100 and 200 slots for undercover DIA operatives to work with Special Forces teams being deployed across North Africa and other trouble spots, officials said.

“Bill McRaven is a very strong proponent of this,” the senior Defense official said.