The United States has conclusive evidence that Syrian President Bashar Assad's regime has used chemical weapons against opposition forces seeking to overthrow the government, crossing what President Barack Obama has called a “red line” that would trigger greater American involvement in the crisis, the White House said.

In response, two U.S. officials said Obama planned to send weapons and ammunition to bolster the rebel forces, though details were unclear. Sen. John McCain, who has been one of the strongest proponents of U.S. intervention in Syria's two-year civil war, said he also had been told of Obama's decision to arm the rebels.

Ben Rhodes, Obama's deputy national security adviser, said Thursday that the president had decided to provide direct “military support' to the opposition but was unable to publicly discuss the type of aid.

“This is going to be different in both scope and scale in terms of what we are providing,” said Rhodes, adding that the additional aid would be aimed at strengthening the effectiveness of the Syria opposition. The U.S. has so far provided the Syrian rebel army with rations and medical supplies.

The 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.

The new U.S. intelligence assessments show that Assad has used chemical weapons, including sarin, on a small scale multiple times in the last year. Up to 150 people have been killed in those attacks, the White House said, 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, announced it had determined that Assad's government had used chemical weapons in the conflict.

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, Assad's most powerful backer. Obama and Putin will hold a one-on-one meeting on the sidelines of the summit, where Obama 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.

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 composition of the defensive military equipment is still being determined in consultation with the opposition military leadership and other nations that are supplying similar material.

Some U.S. 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.

“It is long past time to bring the Assad regime's bloodshed in Syria to an end. As President Obama examines his options, it is our hope he will properly consult with Congress before taking any action,” said Brendan Buck, spokesman for House Speaker John Boehner. (AP)