The United States on Thursday accused Syria of using chemical weapons against the rebels, and announced it would offer "military support" to the forces battling President Bashar al-Assad.

The declaration -- alleging that up to 150 people have been killed in chemical weapons attacks -- means that Syria has crossed what President Barack Obama has called a "red line" in the crisis.

But Washington has not yet decided whether to implement a no-fly zone over Syria, where more than 90,000 people have been killed since the conflict erupted in March 2011, according to the United Nations.

"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," deputy national security adviser Ben Rhodes said in a White House statement.

"The intelligence community estimates that 100 to 150 people have died from detected chemical weapons attacks in Syria to date," he said.

"The president has been clear that the use of chemical weapons -- or the transfer of chemical weapons to terrorist groups -- is a red line for the United States," Rhodes noted.

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

In a conference call with reporters, Rhodes said that Washington had shared its information with Russia about the use of chemical arms, but that Moscow had not yet agreed that Assad should step down.

Rhodes did not say if the United States was moving towards directly arming the rebels battling Assad, but said Obama "will be consulting with Congress on these matters in the coming weeks."

"The United States and the international community have a number of other legal, financial, diplomatic, and military responses available," he said.

"We are prepared for all contingencies, and we will make decisions on our own timeline."

"We're going to act very deliberately," Rhodes said, though noting that both the use of chemical weapons, and the increased involvement of Hezbollah and Iran in the conflict, had "added an element of urgency" to the process.

"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the (rebel) Supreme Military Council. That includes military support," Rhodes said, without offering details.

Senior Republican US Senators John McCain and Lindsey Graham welcomed the administration's move on the issue, but said in a statement that "more decisive actions" were now needed to end the spiraling violence in Syria.

"It's been reported to me by reliable sources that the president... has decided, he's reached the conclusion that they used chemical weapons, and that they are going to provide arms to the rebels," McCain told reporters.

"Since he hadn't announced it, he may reverse that. But I've been told that that's the case."

Earlier, a US defense official said the United States would keep F-16 fighter jets and Patriot anti-missile weapons in Jordan -- which borders Syria -- after a joint military exercise ends this month.