WASHINGTON — President Bush signed a directive this month that expands the intelligence community’s role in monitoring Internet traffic to protect against a rising number of attacks on federal agencies’ computer systems.

The directive, whose content is classified, authorizes the intelligence agencies, in particular the National Security Agency, to monitor the computer networks of all federal agencies — including ones they have not previously monitored.

Until now, the government’s efforts to protect itself from cyber-attacks — which run the gamut from hackers to foreign governments trying to steal sensitive data — have been piecemeal. Under the new initiative, a task force headed by the Office of the Director of National Intelligence (ODNI) will coordinate efforts to identify the source of cyber-attacks against government computer systems. As part of that effort, the Department of Homeland Security will work to protect the systems and the Pentagon will devise strategies for counterattacks against the intruders.

There has been a string of attacks on networks at the State, Commerce, Defense and Homeland Security departments in the past year and a half. U.S. officials and cyber-security experts have said Chinese Web sites were involved in several of the biggest attacks dating back to 2005, including some at the country’s nuclear-energy labs and large defense contractors.

The NSA has particular expertise in monitoring a vast, complex array of communications systems — traditionally overseas. The prospect of aiming that power at domestic networks is raising concerns, just as the NSA’s role in the government’s warrantless domestic-surveillance program has been controversial.

“Agencies designed to gather intelligence on foreign entities should not be in charge of monitoring our computer systems here at home,” said Rep. Bennie Thompson, D-Miss., chairman of the House Homeland Security Committee. Lawmakers with oversight of homeland security and intelligence matters say they have pressed the administration for months for details.

The classified joint directive, signed Jan. 8 and called the National Security Presidential Directive 54/Homeland Security Presidential Directive 23, has not been previously disclosed. Plans to expand the NSA’s role in cyber-security were reported in the Baltimore Sun in September.

A proposal last year by the White House Homeland Security Council to put the Department of Homeland Security in charge of the initiative was resisted by national security agencies on the grounds that the department, established in 2003, lacked the necessary expertise and authority. The tug-of-war lasted weeks and was resolved only recently, several sources said.