The White House has said it is continuing to study assessments by US spy agencies that the Syrian government has used chemical weapons and will not set a timetable for corroborating reports.
"I'm not going to set a timeline, because the facts need to be what drives this investigation, not a deadline," White House spokesman Jay Carney said at a briefing on Friday.
We are continuing to work to build on the assessments made by the intelligence community, that the degrees of confidence here are varying, that this is not an airtight case," he said.
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President Barack Obama said the deployment of chemical weapons by the Syrian government was a "game changer", while also noting that intelligence assessments proving that such weapons had been used were still preliminary.
"Horrific as it is when mortars are being fired on civilians and people are being indiscriminately killed, to use potential weapons of mass destruction on civilian populations crosses another line with respect to international norms and international law," Obama said at the White House.
"That is going to be a game changer. We have to act prudently. We have to make these assessments deliberately. But I think all of us ... recognise how we cannot stand by and permit the systematic use of weapons like chemical weapons on civilian populations," he said.
Two Syrian officials denied the US accusations on Friday, with a senior official saying Damascus did not, and would not, use chemical weapons even if it had them.
Syrian official Sharif Shehadeh called the US claims "lies'' and likened them to false accusations that Iraq possessed weapons of mass destruction ahead of the US-led invasion of that country.
'All options available'
In response to a question, Carney said that President Barack Obama would consider a range of options including, but not exclusive to, military force, should it be determined that Syria has used chemical weapons.
"He retains all options to respond to that, all options," Carney said.
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"Often the discussion, when people mention all options are on the table, everyone just talks about military force.
"It's important to remember that there are options available to a commander in chief in a situation like this that include but are not exclusive to that option."
Earlier on Friday, British Prime Minister David Cameron said there was limited but growing evidence that Syria had committed a war crime by using chemical weapons.
In a cautious assessment mirroring that of Obama's administration, Cameron said the use of chemical weapons was a red line that should trigger greater pressure on President Bashar al-Assad.
"It is limited evidence but there is growing evidence that we have seen too of the use of chemical weapons, probably by the regime," Cameron told the BBC.
"It is extremely serious: this is a war crime ... We need to go on gathering this evidence and also to send a very clear warning to the Syrian regime about these appalling actions," he said.
'Varying degrees of confidence'
On Thursday, US spy agencies said they were investigating reports from Syrian opposition groups that Assad's forces have used sarin gas on at least two occasions during the two-year-old conflict.
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"Our intelligence community does assess with varying degrees of confidence that the Syrian regime has used chemical weapons on a small scale in Syria," Caitlin Hayden, a US National Security Council spokesperson, said.
Hayden said that the US assessment was based in part on "physiological samples" and pointed to the possible use of sarin, a man-made nerve agent used in two attacks in Japan in the 1990s. It can cause convulsions, respiratory failure and death.
However, she said the chain of custody of the weapons was "not clear, so we cannot confirm how the exposure occurred and under what conditions".
Obama has declared that the deployment of chemical weapons would be a game-changer and has threatened unspecified consequences if it happened.
Even so, the Obama administration would likely move carefully, mindful of the lessons of the start of the Iraq war more than a decade ago.
Last month, both the Syrian government and rebels accused each other of using chemical weapons in an attack on the village of Khan al-Assal outside the northern city of Aleppo.
Following the Khan al-Assal attack, the government called for the United Nations to investigate alleged chemical weapons use by rebels.
Syria, however, has still not allowed a team of experts into the country because it wants the investigation limited to the single Khan al-Assal incident while UN Secretary-General Ban Ki-moon is urging the Syrian government to accept an expanded UN probe into alleged chemical weapons use.