President Barack Obama has postponed threatened missile strikes against Syria in a risky gamble that he can win more support for his plan to punish Bashar al-Assad's regime.

To general surprise, the US leader broke with decades of precedent to announce that he would seek approval from Congress for action against Syria's alleged use of chemical weapons.

This effectively pushed military action back until at least September 9, when US lawmakers return from their summer recess.

Obama insisted that he reserves the right to strike regardless of Congress's decision, and a White House official said the pause would also allow him time to build international support.

The Arab League meets in Cairo on Sunday and is expected to condemn Assad, and Obama travels to Russia next week for a G20 Summit that will now be overshadowed by the crisis.

But the toughest battle, and perhaps the most dangerous for Obama's credibility, may yet be with his own former colleagues in Congress, where support for strikes is far from assured.

Indeed, observers warned that he faces the same fate as Prime Minister David Cameron, who on Thursday lost his own vote on authorizing military action in the British parliament.

"The chairman of the joint chiefs has informed me that we are prepared to strike whenever we choose," Obama warned during an address in the White House Rose Garden.

"Our capacity to execute this mission is not time-sensitive. It will be effective tomorrow or next week or one month from now."

At least five US warships armed with scores of Tomahawk cruise missiles have converged on the eastern Mediterranean ready to launch precision strikes on Syrian regime targets.

The FBI has meanwhile increased its surveillance of Syrians living in the United States ahead of a possible US attack and US authorities are also warning of possible retaliatory cyberattacks, The New York Times reported.

And France, which announced its "determination" alongside the US, said it is ready to deploy its own forces in the operation.

Syria, meanwhile, said it has its "finger on the trigger" as it braces for what it had formerly feared was an imminent Western strike.

"The Syrian army is fully ready, its finger on the trigger to face any challenge or scenario that they want to carry out," Prime Minister Wael al-Halqi said.

And the head of Iran's elite Revolutionary Guards warned that Western action would trigger reactions beyond the borders of Tehran's key regional ally.

"The fact that the Americans believe that military intervention will be limited to within Syrian borders is an illusion," said commander Mohammad Ali Jafari.

Shortly before Obama's remarks, a team of UN inspectors left Syria after spending four days investigating last week's alleged chemical attacks on suburbs of Damascus.

The Organization for the Prohibition of Chemical Weapons said that analysis of samples taken at the site would take up to three weeks.

A UN spokesman promised they would give a fair report after conducting these lab tests, but Washington and its allies insist they already know all they need to know.

The Obama administration says it has reliable intelligence that the regime launched a chemical onslaught that killed 1,429 people, including at least 426 children.

President Vladimir Putin of Russia, a close ally of Syria, branded the claims "utter nonsense" and demanded proof.

Syria has denied responsibility for the alleged incident and has pointed the finger of blame at "terrorists" -- its term for the rebels ranged against Assad's forces.

In Damascus, the mood had been heavy with fear, and security forces were making preparations for possible air strikes, pulling soldiers back from potential targets.

Residents were seen stocking up with fuel for generators in case utilities are knocked out by a strike.

The United States, faced with an impasse at the UN Security Council and the British parliament's shock vote, has been forced to look elsewhere for international partners.

Officials said Obama would lobby world powers on the sidelines of next week's St Petersburg G20 summit, while at home the White House was reaching out to lawmakers.

Obama's Democrats control the Senate but the House of Representatives is in the hands of his Republican foes and both sides are divided on the issue, making the outcome uncertain.

In a bid to ease fears of another open-ended war, the White House formally asked Congress for authorization to conduct military strikes in Syria in a draft resolution framing a narrow set of operations.

The document said support from Congress would "send a clear signal of American resolve."

"The objective of the United States use of military force in connection with this authorization should be to deter, disrupt, prevent and degrade the potential for future uses of chemical weapons or other weapons of mass destruction," it added.

Republican Senator Bob Corker, who supports a limited "surgical" strike against Syria, said that Obama should use "every ounce of political capital that he has to sell this."

"I think it is problematic and it could be problematic in both bodies," Corker warned.

In a further complication, hawkish senators John McCain and Lindsey Graham, said they could not support Obama's plan for limited strikes that would not topple Assad.

More than 100,000 people have died since the Syrian conflict erupted in March 2011, and two million have become refugees, half of them children, according to the United Nations.

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