Vice-President Joe Biden has said the US has “no doubt” that the Syrian government has used chemical weapons and that it must be held accountable.
The US has said its military is ready to launch strikes if President Barack Obama order an attack, and allies say they too are ready to act.
The Syrian government has strongly denied claims it used chemical weapons.
UN weapons inspectors are set to return to the site of last week’s suspected attack near Damascus on Wednesday.
Their evidence-gathering visit was delayed by a day after they were fired on.
The US says it will release its own intelligence report into the incident at Ghouta, a suburb of the capital, in the coming days.
More than 300 people reportedly died there.
President Barack Obama is said to have made at least 88 calls to foreign leaders since Wednesday’s suspected attack.
British PM David Cameron said the world could “not stand idly by”, and French President Francois Hollande said France was “ready to punish” whoever was behind the attack.
On Wednesday, Russian Foreign Minister Sergei Lavrov warned that “attempts at a military solution will lead only to the further destabilization” in Syria and the region.
Sergei Lavrov emphasised the need for a political solution in a phone call to the joint UN-Arab League envoy to Syria, Lakhdar Brahimi, the foreign ministry in Moscow said.
Russia, China and Iran have previously warned against launching an attack on the war-ravaged country, where more than 100,000 people are thought to have died in two years of fighting.
Stocks have fallen on global markets and oil prices have shot up amid growing concern about an impending attack.
The US has not yet released its intelligence report into the alleged chemical attack, but US officials now say they are certain the Syrian government was behind the incident.
Joe Biden is the most senior member of the Obama administration to blame the Syrian government for the attack.
In a speech to a veterans’ group in Houston, he said there was “no doubt who was responsible for this heinous use of chemical weapons in Syria: the Syrian regime”.
He said that “those who use chemical weapons against defenceless men, women, and children… must be held accountable”.
White House spokesman Jay Carney earlier said it would be “fanciful” to think anyone else could be responsible – saying the Syrian regime remained in control of the country’s chemical arsenal and used the type of rocket that carried the payload used last Wednesday.
But he insisted there were no plans for “regime change”. Any military campaign is likely to be limited in scope, with missile strikes targeting military sites and no ground troops.
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