US says it will provide military aid to rebels after confirming for first time that it has evidence of nerve gas use

The US has said it will provide military support to the Syrian rebels after confirming it believes there is concrete evidence of nerve gas attacks by government forces against rebel groups.

The assessment that limited attacks have taken place, based on CIA tests on blood, urine and hair samples from dead or wounded rebel fighters, is the first time Washington has supported claims made by British and French intelligence services in recent weeks. Assad has repeatedly denied using any chemical weapons in the bitter civil war.

"Following a deliberative review our intelligence community assesses that the Assad regime has used chemical weapons, including the nerve agent sarin, on a small scale against the opposition multiple times in the last year," said a White House statement.

"Our intelligence community has high confidence in that assessment given multiple, independent streams of information. The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date; however, casualty data is likely incomplete. "

The White House believes its assessment means Syria has crossed the so-called "red line" that President Barack Obama established early in the conflict as a test for further western intervention to support the rebels.

Late on Thursday details began to emerge of the shape military aid might take. Senator John McCain, one of the strongest proponents of US military action in Syria, said he was told on Thursday that Obama had decided to "provide arms to the rebels", a decision confirmed by three US officials, according to the Associated Press. The officials cautioned that decisions on the specific type of weaponry were still being finalised, AP said, but they might include small arms, ammunition, assault rifles and a variety of anti-tank weaponry such as shoulder-fired remote-propelled grenades and other missiles.

The CIA was expected to be tasked with teaching the rebels how to use the arms the White House had agreed to supply, AP said. The New York Times gave a similar outline of the arms involved, while adding that the anti-aircraft munitions hotly sought by the rebels were not under consideration. Syrian rebel groups have repeatedly called for both anti-aircraft and anti-tank missiles.

The deputy national security adviser, Ben Rhodes, said: "The president has made a decision about providing more support for the opposition and will be providing further support to the SMC (Supreme Military Council) and that includes providing military support. I can't detail what types of support yet."

He added: "We have not made any decision about a no-fly zone … The best thing we can do is help the opposition on the ground."

McCain, a leading US hawk who has been pushing for intervention, said: "I applaud the president's decision and I appreciate it."

"But the president of the United States had better understand that just supplying weapons is not going to change the equation on the ground [or] the balance of power. These people – the Free Syrian Army – need weapons, heavy weapons to counter tanks and aircraft, they need a no-fly zone, and Bashar al-Assad's air assets have to be taken out and neutralised. We can do that without risking a single American airplane."

In a conference call with reporters, Rhodes had given examples of the military support the US might be providing but did not mention offensive weapons directly. "We are trying to improve their effectiveness as a fighting force and their cohesion, such as their ability to communicate with each other … and medical equipment to deal with casualties."

Officials in Washington remain divided about more extensive intervention, with many still concerned that enforcing a no-fly zone could backfire or drag the US into further conflict.

National security advisers met in Washington on Wednesday to discuss possible options but Obama was not present.

Thursday's statement based on the CIA reports said there was no reason to think the resistance has access to chemical weapons.

"We believe that the Assad regime maintains control of these weapons, and has taken steps to secure these weapons from theft or attack," it said. "We have no reliable, corroborated reported to indication that the opposition has acquired or used chemical weapons."

The CIA report said the US has acquired blood, urine and hair samples from two Syrian rebels – one dead, and one wounded – who were involved in a firefight with Syrian government forces in mid-March near the town of Utubya, north-east of Damascus.

"While the lethality of these attacks make up only a small portion of the catastrophic loss of life in Syria, which now stands at more than 90,000 deaths, the use of chemical weapons violates international norms and crosses clear red lines that have existed within the international community for decades," said Rhodes.

The White House said it would take decisions on how to proceed "on its own timeline" but would be discussing what to do with allies at the G8 next week.

"The president has said that the use of chemical weapons would change his calculus, and it has," said Rhodes.

"There are a range of options available to us ... but we will make decisions based on the US national interest as well as what might help the rebels on the ground," Rhodes added in the conference call with reporters.

"The influx of foreign fighters to the conflict has added an element of urgency to the situation."

Asked why this decision had been taken now, the White House said it needed to be sure that military assistance would end up in the right hands.

"It takes time to establish a pipeline for direct military assistance but we are now comfortable working with the SMC and General Idriss," said Rhodes. "The type of assistance we are going to provide is going to be substantively different to what we have provided since April."

His comments strongly point toward arming the rebels with significant new weapons.

Rhodes said the new US strategy was "aimed at strengthening the effectiveness of the SMC on the ground and their ability to defend themselves".

Washington said it would be consulting with the United Nations and had provided its alleged evidence of chemical weapons use to the Russians. Obama is due to have bilateral talks with President Putin at next week's G8 summit in Northern Ireland.