Barack Obama appealed to the international community to back his plan to punish Syria with a military strikes, saying the "world had set a red line" over the use of chemical weapons, not him.

Speaking in Stockholm, Obama sought to play down his personal role in the response to the allegations that the Assad regime had gassed its own people in attacks outside Damascus on 21 August.

He also left open the possibility of ignoring a vote against military action in the House of Representatives, saying he did not believe there was a constitutional need for congressional approval.

Although efforts to win over Republican hawks appear close to gaining sufficient support in the Senate, the White House continues to face an uphill struggle to persuade enough members of Congress from both parties to authorise its planned strike against Syria in the House.

But the president claimed there was legal flexibility when asked directly about the possibility of continuing to attack without the full backing of Congress.

"As commander-in-chief I always preserve the right and the responsibilty to act on behalf of America's national security," he said. "I do not believe that I was required to take this to Congress but I did not take this to Congress just because it's an empty exercise. I think it is important to have Congress's support."

He also sought to turn attention to world leaders gathering in St Petersburg for a summit later this week. "The international community's credibility is on the line," he said. "We have to act because if we don't we are effectively saying 'someone who is not shamed can continue to act with impunity'."

Earlier, Vladimir Putin, who will host the G20 summit on Thursday and Friday, warned the US against launching military action in Syria, stating that Russia has "plans" on how it would react if such a scenario unfolded.

In an interview with the Associated Press and Russia's state Channel 1 television, Putin said it was too early to talk about what Russia would do if the US attacked Syria but added: "We have our ideas about what we will do and how we will do it in case the situation develops toward the use of force or otherwise. We have our plans."

At the same time he said Russia did not exclude supporting a UN resolution on punitive military strikes if it were proved that Damascus used poison gas on its own people. But he described the idea that Syrian government forces would use chemical weapons at a time when he said they were in the ascendancy and knowing the potential repercussions as absurd.

In Stockholm, Obama also gave more detail about his unexpected decision on Friday to seek a congressional mandate, saying advice from military commanders that US attacks on Syria would have the same impact in a few weeks time meant there were little harm trying to secure extra political buy-in.

"This had been brewing in my mind for a while," he said. "Had I been in the Senate in the midst of this period, I would probably have suggested to the president that Congress have an ability to weigh in on an issue like this that is not immediate, imminent and time-sensitive."

"It is important for us to get out of the habit of just saying we'll let the president stretch the boundaries of his authority as far as he can and Congress will sit on the sidelines and snipe," added the president.

Crucially, the leadership of both parties have already openly backed the White House plan and Obama's threat to defy any rebellion from more junior members was also endorsed by the most senior Democrat in the House, Nancy Pelosi.

Speaking to reporters on Tuesday, the minority leader said it was a myth that presidents could not defy lawmakers in such circumstances.

"I have been reading what you have written that the president has never gone forward if Congress has opposed the issue," she said. "I remind you that in 1999 President Clinton brought us all together to talk about going to into the Balkans and the vote was 213 to 213 … he went and you know what happened there. I don't think that congressional authorisation is necessary. I do think it's a good thing and I hope we can achieve it."

In Congress on Wednesday, negotiations continued over the wording of Senate and House amendments to the authorisation mandate. Republican hawks were holding out for an amendment that would require the administration to seek to "reverse the battlefield momentum" in favour of rebel forces and involve more sweeping US attacks that would eventually force Assad to step down.

Senator John McCain revealed he would table an amendment on Wednesday afternoon to introduce tougher language to the legislation, which currently focuses only on deterring Syrian use of chemical weapons and "degrading" related military capabilities.

McCain insists he was promised that such regime change would be made part of US policy by President Obama when he met on Monday at the White House with fellow Republican Lindsey Graham.

Obama appeared to respond favourably to McCain's request earlier this week, suggesting that his planned attack on Syria would "fit in" with a wider US policy favouring the departure of Assad.

But many on Capitol Hill are wary of so-called "mission creep" and reluctant to explicitly pursue an Iraq-style strategy of regime change, particularly as this would threaten the chances of getting sufficient Democrats to vote in favour of military authorisation in the House of Representatives.

McCain said he was confident the Senate foreign affairs committee was close to finding a compromise amendment that would include his tougher language. The senator also insisted his plan would not require US troops on the ground.