US President Barack Obama and Russian counterpart Vladimir Putin have failed to end their bitter dispute over US plans for military action in Syria, as half of the G20 called for a "strong" response to a chemical weapons attack blamed on the regime.
Obama said that the world could not "stand idly by" after the Syria chemical attack but Putin warned that it would be "outside the law" to attack without the UN's blessing.
Putin also said Russia would "help Syria" if the US were to strike, pointing to existing military, economic and humanitarian cooperation.
"We spoke sitting down... it was a constructive, meaningful, cordial conversation," Putin said after his previously unscheduled talks with Obama.
"Each of us kept with our own opinion," said Putin, who has emerged as one of the most implacable critics of military intervention against the regime of Syrian President Bashar al-Assad for an alleged chemical weapons attack on August 21.
Asked what Moscow would do if military action began against Syria, Putin said: "Will we help Syria? We will."
The split among leaders of the world's top emerging and developed countries over the issue was symbolised in a statement supported by 11 states at the G20 calling for a "strong international response" to the chemical attack.
Without specifying military action, it said the response would "send a clear message that this kind of atrocity can never be repeated".
The signatories included US allies Britain, France and Saudi Arabia but conspicuously missing were Russia, China and also key EU member Germany.
Washington prepared the ground for possible strikes, evacuating non-essential embassy staff from Beirut and urging Americans to avoid all travel to Lebanon as well as southern Turkey.
The Russian foreign ministry meanwhile Friday strongly warned the United States against targeting Syria's chemical arsenal in any attacks.
--- 'I disagree with his arguments' ---
Putin and Obama spoke for about half an hour on the sidelines of the summit, but neither managed to change the other's mind on Syria.
"He (Obama) disagrees with my arguments, I disagree with his arguments, but we do hear, and we try to analyse," said Putin.
Obama also called the discussion "candid and constructive", adding that it "characterises my relationship with him".
On Friday, the United States said it has come to terms with the fact that no deal could emerge despite repeated attempts at persuading Syria's key ally Russia, and signalled that it would take punitive action against Assad's regime without the UN Security Council's backing.
Obama expressed appreciation for France, saying that he very much valued President Francois Hollande's "commitment to a strong international response for these grievous acts".
He said he would prefer to have an international mandate for the strikes, but that Washington should not be paralysed by a refusal on the part of some countries to act.
"If we're not acting, what does that say?"
Obama, who will address the US nation on Tuesday, is now seeking support from Congress for military action, a process he admitted he always knew was going to be a "heavy lift".
He declined to speculate on what he would do if Congress failed to back intervention, saying he refused to "engage in parlour games now".
Russia and China - both veto-wielding permanent members of the Security Council - have on three occasions voted down resolutions that would have put pressure on Assad.
During a dinner on Thursday, leaders, including Obama, presented their positions on the Syria crisis which only confirmed the extent of global divisions on the issue.
"The gathering was always going to be divided," acknowledged British Prime Minister David Cameron.
Putin said that a majority of countries at the G20 appeared to be supporting his position.
"You said views divided 50-50, that is not quite right," Putin said in answer to a journalist's question, listing only the United States, Turkey, Canada, Saudi Arabia and France as countries supporting an intervention.
UN chief Ban Ki-moon on Friday also warned that military strikes could spark further sectarian violence in the country which he said is suffering from a humanitarian crisis "unprecedented" in recent history.
"I must warn that ill-considered military action could cause serious and tragic consequences, and with an increased threat of further sectarian violence," Ban said.
Hollande said Paris would await the UN inspectors' report on the chemical attack before any Syria strike while Germany urged the release to be sped up.
The Syria crisis and prospect of military intervention has overshadowed the official agenda of the two-day summit of leaders of the world's top economies and emerging markets to stimulate growth and battle tax avoidance.
Several Western states share Putin's opposition to military action and after the British parliament voted against strikes, France is the only power to have vowed it will join American intervention.
The US president held a bilateral meeting Friday morning with President Xi Jinping of China, who like Russia vehemently opposes military action against Syria.
According to US intelligence, more than 1,400 people living in rebel-held suburbs of Damascus were killed in the chemical weapons attack, which involved the use of sarin nerve gas.
With the clock ticking down to strikes, Russia said Syria's Foreign Minister Walid Muallem would travel to Moscow for talks on Monday.