After months of hesitation the White House on Thursday accused the Syrian regime of having crossed a "red line" by using chemical weapons, in a move which could mark a shift in US policy.

Since the uprising against Syrian President Bashar al-Assad erupted in March 2011, President Barack Obama has refused to commit to any US military engagement in the complex conflict, which some warn risks becoming a proxy war.

But with a death toll now over 90,000 and Assad's forces making gains on the ground thanks to help from Hezbollah militants and from Iran, Obama has faced intense pressure both at home and abroad to arm the rebels.

Deputy national security advisor Ben Rhodes delivered the news that the US was going to provide "military support" to the rebel's military command -- but he remained vague on any details.

He said the shift came after an intelligence community assessment 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."

The United States now estimates that between 100 to 150 people have died in at least four separate attacks carried out in March, April and May, Rhodes told reporters.

"As we've consistently said, the use of chemical weapons violates international norms and crosses red lines," he added.

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

So far the US has dribbled out "non-lethal" support to the opposition's Supreme Military Council (SMC), consisting of such things as communications equipment, medical supplies, night-vision goggles, and body armor.

Rhodes said the new assistance would be "different in both scope and scale in terms of what we are providing to the SMC than what we have provided before."

"The president has made a decision about providing more support to the opposition. That will involve providing direct support to the SMC. That includes military support."

He refused to go through an "inventory," but said the aid "would be aimed at strengthening the effectiveness of the SMC on the ground."

And he insisted no decision had yet been made on whether to follow what happened in Libya and set up a no-fly zone -- something analysts have said would be very hard to do given the regime's sophisticated air defenses.

Washington has also shared its information with Russia about the use of chemical arms, Rhodes said, although Moscow had not yet agreed that Assad should step down as part of a parallel political process.

Thursday's moves set the stage for testy talks between Obama and his Russian counterpart Vladimir Putin on the sidelines of a G8 summit in Northern Ireland next week, also expected to focus on the situation in Syria.

Rhodes did not spell out if the United States was moving towards directly arming the rebels, 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.

Senior Republican US Senator John McCain initially welcomed the administration's move, but warned that unless the administration was prepared to arm the rebels it "can't be enough."

"The Russians are providing the most sophisticated equipment -- missiles, airplanes, and we have so far only seen light weapons come in and in our case perhaps flak jackets and MRE's," he said, referring to soldiers' rations.

"I had been told that as I mentioned on the floor that it had been military assistance, but they need a lot more than military assistance. We need to establish the no-fly-zone. We need a safe zone within Syria."

Rhodes stressed the United States was still working towards a political settlement, seeking to bring together the regime and the opposition for peace talks in Geneva.

"In our view, that process would have to involve Bashar al-Assad stepping down, and the Russians have not yet agreed to it," he added.