The debate over arming Syrian rebels has intensified after the US declared that it had conclusive evidence the Assad regime had crossed "clear red lines" by deploying chemical weapons.
President Barack Obama has made clear such proof would trigger greater American involvement and the White House indicated it would further increase the "scope and scale" of assistance to opposition forces, including direct "military support".
There was no immediate announcement of what that would comprise or the timescale for accelerating support.
"The United States and the international community have a number of other legal, financial, diplomatic, and military responses available. We are prepared for all contingencies, and we will make decisions on our own timeline," deputy national security adviser Ben Rhodes said.
The UK - along with France - has reported repeated small-scale incidents of apparent chemical weapon use by forces loyal to Syrian president Bashar Assad to the United Nations investigation team.
Both countries led a successful joint bid to lift an EU embargo on supplying arms to opposition forces although Prime Minister David Cameron stressed on Thursday that no decision had been taken to arm the rebels.
He is due to hold talks with Russian president Vladimir Putin at Downing Street on Sunday as part of efforts to break the international deadlock. The crisis will also be high on the agenda when they join other world leaders, including Mr Obama and French president Francois Hollande, at the G8 summit in Northern Ireland on Monday.
Foreign Secretary William Hague discussed the crisis - which the United Nations now believes has cost at least 93,000 lives - with US counterpart John Kerry on Wednesday. He said after those talks that the international community must be "prepared to do more" to up the pressure on the regime.
A Downing Street spokeswoman said: "There is a growing body persuasive evidence showing that the regime used - and continues to use - chemical weapons, including sarin. The room for doubt is diminishing. Assad must grant the UN investigation unrestricted access to investigate on the ground in Syria and establish the full facts."
Mr Rhodes said US intelligence services had "high confidence" that weapons including the nerve agent sarin had been used on a small scale on multiple occasions - causing at least 100-150 deaths. There was "no reliable, corroborated reporting" to back claims the rebels either possessed or had used them, he said.