Tensions over military action in Syria are expected to dominate G20 talks when they begin today in Russia.
US President Barack Obama has warned that the credibility of the international community is at risk and said it could not "be silent" in the face of the Syrian regime's "barbarism".
The summit in St Petersburg comes against a backdrop of frosty relations between the US and Russia over Syria and other issues, such as whistleblower Edward Snowden.
A planned one-on-one meeting ahead of the G20 between President Obama and Russian President Vladimir Putin was scrapped when it became clear the pair would not find common ground over the alleged chemical attacks.
The seating plan at the summit - which is intended to focus on global economic issues - has also reportedly been changed to put physical distance between the two leaders.
Speaking ahead of the G20, the US President said he had "hit a wall" with Mr Putin, who has warned that the use of force without United Nations approval would be an "aggression" and a violation of international law.
Mr Putin has said he will only support action if very strong evidence shows that the Syrian regime was behind the alleged August 21 gas attacks.
He threw another wrench into relations on Wednesday by accusing US Secretary of State John Kerry of lying.
He said Mr Kerry's denial that an al Qaeda-linked group is fighting with the Syrian opposition was "very unpleasant and surprising for me".
"He is lying and he knows that he is lying. This is sad," said Mr Putin.
Meanwhile, John Kerry addressed the US Senate's Foreign Relations committee last night. Afterwards they voted in favour of backing military action against Syria:
Former US ambassador to Syria, Richard W. Murphy told Newstalk's Breakfast Syria has given a clear warning that it will strike back against Turkey, Jordan and Israel if it is hit:
The US claims more than 1,400 people died in the alleged gas attacks on the outskirts of Damascus, although other countries such as France have said the number is lower.
British Prime Minister David Cameron has said he will push for a tough international response at the G20 summit, but risks being side-lined after losing a Commons' vote over UK military involvement.
He is not expected to have a formal meeting with Mr Obama, whose efforts to win domestic approval for US military involvement edged closer on Wednesday after a key Senate committee voted to support a draft resolution.
The proposal limits initial strikes to 60 days but rules out "boots on the ground" and is now set for a vote in the wider Senate, likely to be next week.
French MPs have also been debating the prospect of supporting any US strikes, but won't vote on the subject.
President Francois Hollande has the power to order short military action without parliamentary approval, but he has said he will await the US Senate vote before acting.
Meanwhile, one of Syrian President Bashar al Assad's advisers has accused the US of using "lies" and "fabrications" to justify the use of military force.
Syrian reaction
Dr Bouthaina Shaaban says: "They claim to want to be targeting weapons in exactly the same way they claimed to be targeting weapons of mass destruction in Iraq.
"They are using the same lies, the same fabrications, the same claims, in order to target our country and our people."
