Barack Obama left a fractious G20 summit in St Petersburg on Friday after assembling a fragile alliance of countries accusing Bashar al-Assad of being responsible for using poison gas against civilians. However, the US president left behind a defiant Russian counterpart threatening unspecified military support for Syria if America attacks.

Vladimir Putin claimed that a majority of the G20 opposed any US-led intervention, and gave no ground by continuing to insist that the chemical weapons attacks were a provocation by Syrian rebels designed to win international backing for an attack on the Assad regime. David Cameron described Putin's position as impossible.

Putin revealed that he and Obama had had a one-to-one meeting lasting around 30 minutes in which they had discussed Syria. Both men had listened to the other's position but they had not agreed, he said.

British sources suggested that Obama, struggling to put together a majority in the US Congress for military strikes, may have to wait for up to a fortnight for a vote in the House of Representatives, where opposition is strong.

Echoing that timing, the French president, François Hollande, the only definite European supporter of a military strike, said he did not expect a congressional vote in the US until the UN weapons inspectors had reported on whether there had been a chemical attack on 21 August. Cameron added that no one doubted there had been an attack, not even Syria; the dispute was over culpability, he said.

In a minor diplomatic advance for Obama, 11 of the G20 nations signed a joint statement at the end of the two-day summit calling for "a strong international response to a grave violation of the world's rules" in response to last month's chemical weapons attack in Ghouta, east of the Syrian capital, Damascus.

The signatories, including the UK, the US and France, said evidence "points clearly to the Syrian government being responsible for the attack which is part of a pattern of chemical weapons use by the regime" and warned it would not be possible to achieve a UN consensus on action.

The signatories also "recognise that the UN security council remains paralysed, as it has been for two and a half years. The world cannot wait for endless failed processes that can only lead to suffering in Syria. We support efforts by the US and other countries to reinforce the prohibition on the use of chemical weapons."

The painfully constructed wording stops short of explicit support for a punitive, but limited, military strike by the US. Yet the statement represents more international sympathy than seemed likely at the summit's outset. Other signatories included Australia, Canada, Italy, Japan, South Korea, Saudi Arabia, Spain and Turkey – a coalition that may sway some US congressmen weighing up whether to defy domestic America opinion and back military strikes. A Downing Street source claimed the statement "backs US efforts and the American president has clearly set out his intended military response".

Russia, China, South Africa, Indonesia, Argentina and Brazil were among those that refused to sign. But it was the absence of German chancellor Angela Merkel's signature that was the most frustrating – a result deemed to be a blow to the Franco-German alliance.

Obama, who will address the American people next Tuesday in a televised address, was equivocal on whether he would persuade Congress. "It's conceivable at the end of the day I don't persuade a majority of the American people that it's the right thing to do," he said. "And then each member of Congress is going to have to decide."

The president said he could ignore a rejection of military action by Congress, but hinted such defiance would be hard to justify. A resolution is likely to be voted upon in the Senate on Wednesday after it was formally introduced on Friday. Obama said during remarks at the end of the summit that he put the issue before Congress "because I could not honestly claim that the threat posed by Assad's use of chemical weapons on innocent civilians and women and children posed an imminent, direct threat to the United States".

"The majority of the room is comfortable with our conclusion that Assad, the Assad government, was responsible for their use," he said, adding that this was disputed by Putin.

A number of countries believed that any military force needed to be decided at the UN security council (UNSC), a view he said he did not share. "Given security council paralysis on this issue, if we are serious about upholding a ban on chemical weapons use then an international response is required, and that will not come through security council action," Obama said. That view was shared by Cameron, who argued that world morality could not be "contracted out to the UNSC".

Putin offered a different interpretation of the state of world opinion at his closing press conference. He said: "Will we be helping Syria? We will. And we are already helping – we send arms, we co-operate in the economic sphere."

In many of the private sessions, the Russian president has appeared agnostic on whether the poison gas was used by Assad's forces or rebels. But in public he took a harder line: "I presume that everything concerning the so-called use of chemical weapons is a provocation on the part of the fighters, who expect assistance from the outside, I mean assistance from the countries that have supported them from the very start. This is the essence of this provocation."

He went to argue that the use of force against Syria would be illegitimate. "The use of force on a sovereign state is only possible if it is done for self-defence – and as we know Syria is not attacking the US – or under a decision made by the UN security council," Putin said. "As one participant in our discussion said, those who act otherwise put themselves outside of law."

He said it was not true to assert opinion had been 50-50 divided at the summit. He claimed only Turkey, Canada, Saudi Arabia and France supported military operations against Syria, while Russia, China, India, Indonesia, Argentina, Brazil, South Africa and Italy opposed the option at the summit. Russia also warned the US and its allies against striking any chemical weapon storage facilities in Syria. The Russian foreign ministry said such targeting could release toxic chemicals and give militants or terrorist access to chemical weapons.

"This is a step toward proliferation of chemical weapons not only across the Syrian territory but beyond its borders," the Russian statement said.

The Kremlin said on Friday that Russia was boosting its naval presence in the Mediterranean, moving warships into the area and stoking fears about a larger international conflict if the United States orders air strikes.

Illustrating the risks associated with a strike, the US state department ordered non-essential American diplomats to leave Lebanon, a step under consideration since last week when Obama said he was contemplating military action against the Syrian government.