US Vice-President Joe Biden has said there is "no doubt" that the Syrian government has used chemical weapons and that it must be held accountable, SIA reports citing BBC.
The US has said its military is ready to launch strikes should 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 Obama is said to have made at least 88 calls to foreign leaders since Wednesday's suspected attack, and spoke to UK Prime Minister David Cameron for the second time on Tuesday.
Mr 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.
The UK's National Security Council is due to convene on Wednesday to discuss possible responses, while Parliament is to be recalled on Thursday.
On Wednesday, Russian Foreign Minister Sergei Lavrov warned that "attempts at a military solution will lead only to the further destabilisation" in Syria and the region.
Mr 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.
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