Barack Obama and Vladimir Putin set for G-20 showdown over Syria

President Barack Obama is expected to make his case for military strikes in Syria to world leaders at the G-20 summit Thursday, despite host nation Russia warning that without U.N. backing, such a move would be an act of aggression.

While not on the official agenda in St. Petersburg, the crisis in Syria and proposed punitive action by the U.S. in response to the alleged use of chemical weapons by the government of President Bashar al-Assad is likely to dominate proceedings, with a series of sideline meetings planned on the issue.

Obama, who on Wednesday cleared a major domestic hurdle to strikes by winning the support of the Senate Foreign Relations Committee, will now attempt to bridge deep divisions in the international community.

Speaking during a trip to Stockholm, the U.S. president said the world had set "a red line" for Syria and it could not now remain silent in the face of the alleged chemical weapons attack on Damascus suburbs.

But Russian President Vladimir Putin, a fierce opponent of the proposed military action, warned on the eve of the summit that it would be unacceptable for the West to go ahead with punitive strikes without first getting U.N. Security Council approval.

The Kremlin demanded "convincing" proof that the Assad government was responsible for using chemical weapons against its own people.

According to U.S. intelligence, more than 1,400 people living in rebel-held suburbs of Damascus were killed in the strike, which is alleged to have involved the use of sarin nerve gas. But that death toll is higher than other estimates, with the British-based Syrian Observatory for Human Rights saying that it had compiled a list of 502 victims.

Beyond convincing Russia, Obama is also expected to have a tough time selling the idea of military strikes to other nations. China, another veto-wielding Security Council member, has already expressed its "grave concerns" over unilateral military strikes.

German Chancellor Angela Merkel has repeatedly ruled out her country's participation in any U.S.-led military strike against Assad's government, while the British parliament also has rejected the idea. 

But speaking in Sweden Wednesday, Obama said: "I didn't set a red line. The world set a red line," referring to international rules banning the use of chemical weapons, even in case of war.

"My credibility is not on the line," he said. "The international community's credibility is on the line, and America and Congress's credibility is on the line."

The Syria conflict threatens to overshadow other items on the G-20 agenda -- such as an "action plan" for sustainable and balanced global growth -- even though it has not been formally penciled in for discussion.

White House officials have said Obama will hold meetings on the sidelines of the summit with French President Francois Hollande, one of the U.S.'s main backers over a proposed strike on Syria, as well as the leaders of China and Japan.

While no formal bilateral meeting is planned with Putin, a White House official suggested there would likely be some kind of dialogue.

Ties between Russia and U.S. have sunk to a new in recent months even beyond the deep-seated divisions over Syria. Washington has also been angered by Moscow's granting of asylum to U.S. fugitive intelligence leaker Edward Snowden and a string of Russian laws targeting non-governmental organizations and opposition rallies.

In a further sign of bilateral tensions, John Boehner, Speaker of the U.S. House of Representatives, rejected a request to meet a Russian delegation to discuss Syria, his spokesman said Wednesday.

In an interview with the Associated Press and Russian state media, Putin said that did not rule out agreeing to strikes against Syria, but only if they are sanctioned by the U.N. Security Council.

"Any other pretext or method which might be used to justify the use of force against an independent sovereign state is inadmissible and can only be interpreted as an aggression,” he added.

He later told members of the board of human rights in the Kremlin that "only the U.N. Security Council can give approval for the use of force against another state."

The United Nations is making a desperate new push for a Syria peace conference even as the United States prepares a possible military strike, according to diplomats.

Talks on a proposed conference are to be re-launched at the G-20 summit, envoys said.

Meanwhile, Syrian Deputy Foreign Minister Faisal Muqdad told Agence France-Presse that his government was ready to retaliate in case of foreign military action.

"The Syrian government will not change position even if there is World War III. No Syrian can sacrifice the independence of his country," Muqdad said. "Syria has taken every measure to retaliate against... an aggression."

U.S. military action against Syria seemed imminent last week, but Obama has deferred the move, and is seeking congressional backing in a vote scheduled for Monday.

The Senate Foreign Relations Committee on Wednesday gave its backing by a 10-7 vote for the use of force. Senate leaders said the full chamber will vote next week on the motion, with approval expected.

The House of Representatives will also begin its deliberations next week.

Al Jazeera and wire services