British Prime Minister David Cameron has echoed US defence secretary Chuck Hagel in alleging that Syrian government troops may have used chemical weapons.
The UK Prime Minister said "there is limited bit growing evidence" that chemical weapons, possibly sarin gas, have been use by the Syrian army against its own people - and this may constitute a "red line" leading to intervention by the western power.
Cameron told the BBC that the use of chemical weapons "is a war crime, and we should take it very seriously.
"'We're trying to consider the evidence with our allies, make sure that we can verify it, but this is extremely serious, and I think what President Obama said was absolutely right - that this should form for the international community a red line for us to do more."
Earlier, France and a senior official in the Israel Defence Force made similar claims, maintaining Assad had used "unconventional weapons" against those attempting to depose him on several occasions.
Israel's Brig Genl Itai Brun said: "[Assad's] regime used lethal chemical weapons against militants in a series of incidents over recent months."
Earlier this week, two letters from the White House confirmed there is evidence of at least two chemical weapons attacks in Syria.
"Our intelligence community does assess, with varying degrees of confidence, that the Syrian regime has used chemical weapons on a small scale in Syria, specifically, the chemical agent sarin," say the letters, which were signed by Barack Obama and addressed to two senators.
The information was made public by Hagel, who announced it to reporters traveling with him in the United Arab Emirates.
"It violates every convention of warfare," the defence secretary added.
No easy choice
Obama had declared that the use of chemical weapons in the two-year civil war would be "game changer" that would cross a "red line" for a major military response. However both the US and the UK said further evidence is needed.
"We need to go on gathering this evidence and also to send a very clear warning to the Syrian regime about these appalling actions," Cameron said.
The PM said he did not "want to see" British troops sent in to Syria, and other ways of increasing the pressure on Assad are being explored.
"The question is how do we step up the pressure and, in my view, what we need to do - and we're doing some of this already - is shape that opposition, work with them, train them, mentor them, help them, so that we put the pressure on the regime and we can bring this to an end," Cameron said.
"It is extremely difficult though, and extremely frustrating."
'The least flawed option'
The main difficulty is posed by the presence of extremist groups amid the rebel forces, such as Jabhat al-Nusra, which recently declared its affiliation to al-Qaida and has been designated as a terrorist group by the US.
Arming the rebels is a "lot harder that it was before," said Republican senator Lindsey Graham.
"We've gotten to the point now where the opposition has been affected by the radicals," Graham said.
"Right weapons in right hands is the goal. The second war is coming. I think we can arm the right people with the right weapons. There's a risk there, but the risk of letting this go and chemical weapons falling into radical Islamists' hands is the greatest risk."
"There's no easy choice here," said US Democratic Senator Claire McCaskill, a member of the Armed Services Committee. "All the alternatives are flawed. It's just finding the least flawed among them that will get Assad out."
Creating a no fly zone over Syria is made difficult by the regime's robust air defence system, which includes as many as 300 mobile surface-to-air missile and defence systems, and more than 600 static missile launchers and sites, according to a report by the Institute for the Study of War.
A group of US lawmakers, including senator and former Presidential candidate John McCain, advocate the creation of a safe zone inside Syria, along its border with Turkey.
"You can establish it [the safe zone] by taking out their aircraft on the ground with cruise missiles and using the Patriot [missile] also. No American manned aircraft in danger," McCain said.
So far support for the rebel groups has been limited to non-lethal aid.