People protest against proposed US military action against Syria, in Times Square in New York, Aug 31, 2013. [Photo/Agencies]

Decision made Friday night

At the White House, Obama's decision surprised senior aides when he informed them of it on Friday night after they had concluded the president already had the legal authority to act on his own.

Officials said he laid out the idea of going to Congress during a 45-minute walk on the White House south grounds with chief of staff Denis McDonough, then debated the risks with others in his inner circle, some of whom argued against his logic.

Senior administration officials who briefed reporters after Obama spoke said they believed Congress will vote in favor of a US military strike because of the threat chemical weapons pose to the security of US ally Israel and other friends in the region.

Jon Alterman, a former State Department official who is a Middle East expert at the Center for Strategic and International Studies, said failure by Obama to get his way "could be something that not only dominates September and October, but could shadow the rest of the administration."

Obama sent draft legislation to Congress on Saturday formally asking for approval to use military force in Syria to "deter, disrupt, prevent and degrade" the potential for further chemical attacks.

The Aug 21 attack was the deadliest single incident of the Syrian civil war and the world's worst use of chemical arms since Iraq's Saddam Hussein gassed thousands of Kurds in 1988.

The team of UN experts arrived in the Netherlands on Saturday carrying evidence and samples relating to the attack. They had flown from Beirut after crossing the border into Lebanon by road earlier in the day.

The 20-member team had arrived in Damascus three days before the attack to investigate earlier accusations of chemical weapons use. After days holed up in a hotel, they visited the sites several times, taking blood and tissue samples from victims and from soldiers at a government hospital.

War weariness cost Washington the support of its closest ally: Britain has voiced backing for action but was forced to drop any plans for a military strike after Cameron unexpectedly lost a vote over it in parliament on Thursday, straining London's "special relationship" with Washington.

Syria and its main ally, Russia, say rebels carried out the gas attack as a provocation. Moscow has repeatedly used its UN Security Council veto to block action against Syria and says any attack would be illegal and only inflame the civil war there.

"I am convinced that (the chemical attack) is nothing more than a provocation by those who want to drag other countries into the Syrian conflict," Russian President Vladimir Putin said on Saturday.

Syria's Foreign Ministry repeated its denial that the government had used chemical weapons against its own people. Washington says the Syrian denials are not credible and that the rebels would not have been able to launch such an attack.

Syria's neighbour Turkey backs the use of force. The Arab League has said Syria is to blame for the chemical attack but so far stopped short of explicitly endorsing Western military strikes. Arab League foreign ministers are due to meet in Cairo on Sunday.

Iran, Assad's main ally in the region, has condemned plans for strikes and warned of wider war.

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