Obama: International community cannot remain silent over Syria
Barack Obama has said the international community's credibility is at stake over the response to Syria's chemical weapons attack.
His top advisers meanwhile took the argument for action to the opposition-controlled House of Representatives, where the significant support seen in the Senate will be harder to find.
Asked about his past comments drawing a "red line" against the use of chemical weapons, Mr Obama said that line had already been drawn by a chemical weapons treaty ratified by countries around the world. "That wasn't something I made up," he said.
With Mr Obama in Europe for a G20 summit, his national security aides were facing public and private hearings in Washington to argue for Congress' authorisation for strikes against president Bashar Assad's regime.
The Senate Foreign Relations Committee had been expected to vote first on the use of force, followed by other debates as the president's request makes its way through Senate and House committees before coming before the two chambers for a final vote.
But the Senate committee's public meeting was delayed after senator John McCain, an outspoken advocate of intervention against Assad's regime, said he does not support a new Senate resolution which permits Mr Obama to order a "limited and tailored" military mission against Syria, as long as it does not exceed 90 days and involves no US troops on the ground for combat operations.
The administration also needs to persuade a Republican-dominated House of Representatives that has opposed almost everything on Mr Obama's agenda since the party seized the majority more than three years ago. The top opposition Republican in Congress, House Speaker John Boehner, has signalled key support, saying the US has "enemies around the world that need to understand that we're not going to tolerate this type of behaviour."
Mr Obama was asked if he would take action against Syria if he fails to get approval from Congress. As commander in chief, "I always preserve the right and the responsibility to act on behalf of America's national security," he said.
Secretary of State John Kerry, Defence Secretary Chuck Hagel and the chairman of the Joint Chiefs of Staff, General Martin Dempsey, were trying to make their case in a public hearing before the House Foreign Affairs Committee. They and other senior administration officials also were providing classified briefings to the Senate Foreign Relations and Armed Services committees.
But even supporters of military action urged Mr Obama to do more to sell his plans to an American public that is highly sceptical after a decade of war in Iraq and Afghanistan.
Mr Obama is expected to find little international support for action. Among major allies, only France has offered publicly to join the United States in a strike. The United Nations secretary-general has warned that any "punitive" strike on Syria would be illegal without a sound case for self-defence or the approval of the Security Council, where Syria ally Russia has used its veto power to block action against Assad's regime.
Russian president Vladimir Putin has warned the West against taking one-sided action in Syria but said Russia "doesn't exclude" supporting a UN resolution on punitive military strikes if it is proved that Syria used poison gas on its own people.