Obama has repeatedly said the use of chemical weapons would cross a "red line," suggesting it would trigger greater American intervention in the two-year crisis.
Sen. John McCain, R-Ariz., one of the strongest proponents of U.S. military action in Syria, said he was told Thursday that Obama had decided to "provide arms to the rebels," a decision confirmed by three U.S. officials. The officials cautioned that decisions on the specific type of weaponry were still being finalized, though the CIA was expected to be tasked with teaching the rebels how to use the arms the White House had agreed to supply.
Still, the White House signaled that Obama did plan to step up U.S. involvement in the Syrian crisis in response to the chemical weapons disclosure.
"This is going to be different in both scope and scale in terms of what we are providing," said Ben Rhodes, Obama's deputy national security adviser.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies.
Thursday's announcement followed a series of urgent meetings at the White House this week that revealed deep divisions within the administration over U.S. involvement in Syria's civil war. The proponents of more aggressive action — including Secretary of State John Kerry — appeared to have won out over those wary of sending weapons and ammunition into a war zone where Hezbollah and Iranian fighters are backing Assad's armed forces, and al-Qaida-linked extremists back the rebellion.
Obama still opposes putting American troops on the ground in Syria and the U.S. has made no decision on operating a no-fly zone over Syria, Rhodes said.
U.S. officials said the administration could provide the rebels with a range of weapons, including small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired remote-propelled grenades and other missiles. However, a final decision on the inventory has not been made, the officials said.
Most of those would be weapons the opposition forces could easily use and not require much additional training to operate. Obama's opposition to deploying American troops to Syria makes it difficult to provide much large-scale training. Other smaller- scale training can be done outside Syria's borders.
A U.S. official said the CIA and special operations trainers have already been training Syrian rebels on the use of anti-aircraft weaponry provided by the Persian Gulf states, as well as encrypted communications equipment, and was expected to run the expanded training program as well.
All of the officials insisted on anonymity in order to discuss internal administration discussions.
Word of the stepped up assistance followed new U.S. intelligence assessments showing that Assad has used chemical weapons, including sarin, on a small scale multiple times in the last year.
U.S. intelligence estimates 100 to 150 people have been killed in those attacks, the White House said, constituting a small percentage of the 93,000 people killed in Syria over the last two years.
The White House said it believes Assad's regime still maintains control of Syria's chemical weapons stockpiles and does not see any evidence that rebel forces have launched attacks using the deadly agents.
The Obama administration announced in April that it had "varying degrees of confidence" that sarin had been used in Syria. But they said at the time that they had not been able to determine who was responsible for deploying the gas.
The more conclusive findings announced Thursday were aided by evidence sent to the United States by France, which, along with Britain, has announced it had determined that Assad's government had used chemical weapons.
Obama has said repeatedly that the use of chemical weapons would cross a "red line" and constitute a "game changer" for U.S. policy on Syria, which until now has focused entirely on providing the opposition with nonlethal assistance and humanitarian aid.
The White House said it had notified Congress, the United Nations and key international allies about the new U.S. chemical weapons determination. Obama will discuss the assessments, along with broader problems in Syria, next week during the G-8 summit in Northern Ireland.
Among those in attendance will be Russian President Vladimir Putin, one of Assad's most powerful backers. Obama and Putin will hold a one-on-one meeting on the sidelines of the summit, where the U.S. leader is expected to press his Russian counterpart to drop his political and military support for the Syrian government.
"We believe that Russia and all members of the international community should be concerned about the use of chemical weapons," Rhodes said.
Britain's U.N. Ambassador Mark Lyall Grant said his country was "not surprised by the determination made by the U.S. government," given its own assessments, and was in consultation with the Americans about next steps.
The U.S. has so far provided the Syrian rebel army with rations and medical supplies. In April, Kerry announced that the administration had agreed in principle to expand its military support to the opposition to include defensive items like night vision goggles, body armor and armored vehicles.
The Syrian fighters have been clamoring for bolder Western intervention, particularly given the estimated 5,000 Hezbollah guerrillas propping up Assad's forces. Assad's stunning military success last week at Qusair, near the Lebanese border, and preparations for offensives against Homs and Aleppo have made the matter more urgent.
While McCain has pressed for a greater role for the U.S. military, other lawmakers have expressed reservations about American involvement in another conflict and fears that weapons sent to the rebels could fall into the hands of al-Qaida-linked groups.
Sen. Chris Murphy, D-Conn., a member of the Senate Foreign Relations Committee, condemned the Assad regime but expressed serious concerns about the United States being pulled into a proxy war.
"There are many actions that the United States can take to increase our humanitarian assistance to refugee populations and opposition groups short of injecting more weapons into the conflict," Murphy said. "I urge the president to exercise restraint and to consult closely with Congress before undertaking any course of action to commit American military resources to Syrian opposition forces."
House Speaker John Boehner, R-Ohio, also urged the White House to consult with Congress.
"It is long past time to bring the Assad regime's bloodshed in Syria to an end," he said through a spokesman, Brendan Buck. "As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action."
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