A protester waves signs as Chairman of the Joint Chiefs General Martin E. Dempsey, John Kerry, US Secretary of State, and Chuck Hagel, Secretary of Defense, arrive to present the administration's case for US military action against Syria to a Senate Foreign Relations Committee hearing in Washington September 3, 2013. [Photo/Agencies]

Obama has said he is "comfortable going forward without the approval of a United Nations Security Council that so far has been completely paralyzed and unwilling to hold Assad accountable.

Syrian UN Ambassador Bashar Ja'afari had sharp words for Obama's administration after a closed-door meeting between UN disarmament chief Angela Kane and the 37 UN member states that asked Ban to investigate the August 21 poison gas attack.

"Who asked Mr. Obama to be the bully of the world?" Ja'afari said.

Obama was due to leave Washington on Tuesday for a G20 meeting in Russia. France said foreign ministers of some of the G20 member states will convene on the sidelines of the meeting to discuss Syria.

The conflict has divided the Middle East on sectarian lines, with Shi'ite Iran backing Assad and Washington's Sunni Arab Gulf allies supporting the mainly Sunni rebels. It has also revived Cold War-style tensions between the Western powers and Moscow.

In an interview in Le Figaro, Assad told the French newspaper: "Everybody will lose control of the situation when the powder keg blows. There is a risk of a regional war."

The influential pro-Israel lobby group AIPAC urged US lawmakers to approve a resolution authorizing strikes to punish Assad. "This is a critical moment when America must also send a forceful message of resolve to Iran and Hezbollah - both of whom have provided direct and extensive military support to Assad," AIPAC said in a statement.

The rebels have been struggling to hold ground in recent months, let alone advance. According to one opposition report, government forces took the strategic northwestern town of Ariha on Tuesday, though others said the battle was not over.

While Obama's wait for Congress to return from its summer recess seems to rule out Western military action this week, Israeli forces training in the Mediterranean with the US Navy set nerves on edge in Damascus with a missile test.

When Moscow raised the alarm that its forces had detected the launch of two ballistic "objects" in the Mediterranean, thoughts of a surprise strike on Syria pushed oil prices higher.

Clarification came only later when the Israeli Defence Ministry said that its troops had - at the time of the Russian alert - fired a missile that is used as a target for an anti-missile defence system during an exercise with US forces.