“Speeches by key leaders at the end of the G20 summit in St Petersburg have laid bare the bitter divisions over possible military action in Syria.
Russian President Vladimir Putin restated his opposition to any strike, saying it would destabilise the region.
A joint statement from the US and 10 other nations called for a strong international response.
The US government accuses President Bashar al-Assad’s forces of killing 1,429 people in a poison-gas attack in the Damascus suburbs on 21 August. Mr Assad has blamed rebels for the attack.
China and Russia, which have refused to agree to a UN Security Council resolution against Syria, insist any military action without the UN would be illegal.
Mr Putin said the discussions about Syria on Thursday evening had gone on well past midnight. He added that he had had a one-to-one meeting with Mr Obama in which they had discussed Syria. Both men had listened to the other’s position but had not agreed, he said.
Mr Putin said he believed a majority of the populations in countries supporting military action were against it.
Meanwhile French President Francois Hollande, who has been a firm proponent of intervention, said he would await for a report from UN weapons inspectors before taking a decision on military action. The inspectors’ findings are not due to be made public until the week beginning 15 September – possibly even later.
Also speaking at the end of the summit, Mr Obama said there was a “unanimous” view that chemical weapons had been used in Syria. He also said most leaders present at the summit thought it was most likely that the regime of Mr Assad was responsible. Mr Obama argued action was required even when the Security Council was paralysed, as the international consensus against the use of chemical weapons had to be upheld.
However, Mr Putin described the use of chemical weapons as “a provocation on the part of the militants who are expecting to get support from outside”.
British Prime Minister David Cameron said the UK had “made available additional evidence of cloth and soil samples which underline the now overwhelming picture of a war crime” on 21 August. Mr Cameron said that, given the depth of international divisions, the “summit was never going to reach agreement”. But he added that if there were only a response to the crisis through the UN Security Council, that would mean the UK “contracting out its morality and foreign policy to the potential of a Russian veto”.
While the UK, Canada and Turkey all support Mr Obama’s call for action, the only leaders at the G20 meeting to commit to force in Syria are the US and France.
Correspondents in St Petersburg say opponents of US military intervention appear to far outnumber supporters within the G20.
A joint statement from the US, Australia, Canada, France, Italy, Japan, Republic of Korea, Saudi Arabia, Spain, Turkey and the UK called for “a strong international response” over Syria.
Recognising that the UN Security Council had been paralysed over the issue, the statement added the “world cannot wait for endless failed processes that can only lead to increased suffering in Syria and regional instability”.
In his comments to reporters on Friday, Mr Obama did not make clear what he would do if the US Congress decided against military action in a vote expected next week.
A poll commissioned by the BBC and ABC News suggested more than one-third of Congress members were undecided whether or not to back military action – and a majority of those who had made a decision said they would vote against the president.
The US ambassador to the UN Samantha Power said that the US had “exhausted the alternatives” to military action.
She said that according to American estimates, the 21 August attack had “barely put a dent” in Syria’s chemical weapons stockpile.
Also on Friday, the UN appealed for more aid for people in Syria, and also for the estimated two million Syrians who have fled their country.
UN humanitarian chief Valerie Amos told the BBC that donor countries should “look again” at their contributions and be “as generous as they can”.
Mr Cameron announced on Friday that some countries at the G20 had agreed measures to speed up the delivery of aid, including lifting bureaucratic obstacles such as custom rules.
He said earlier that the UK would give an additional £52m ($80m) in aid for Syria – much of it for medical training and equipment to help civilians targeted by chemical attacks.
However, correspondents point out that the delivery of aid is complicated by the need to negotiate with armed groups on the ground.
Meanwhile, on the ground in Syria, rebels have withdrawn after briefly entering an ancient Christian town north of Damascus, the main opposition alliance has said. Free Syrian Army (FSA) units captured military positions outside Maaloula after heavy clashes with government forces and militiamen on Thursday.
Also on Friday, the US embassy in Lebanon said that it would be evacuating its non-essential staff.
The move was prompted by “threats to US mission facilities and personnel,” a statement said. The US Consulate General in the Turkish city of Adana is also withdrawing non-essential staff”.
Everybody is expecting UN report that would clear the clouds and give a new perspective over Syria’s tense. France and US are the only nations that want to react fast in order to stop any more attacks from the Assad regime. As long as UN has the universal power of deciding the faith of a country and the right to call upon an attack against one all the leaders of the world will have to wait. Evidence of the chemical attack will soon be brought to light and if it will be proofed that the leader of Syria is to be blamed for the thousands of dead people, the war will break.