Western powers could attack Syria within days, envoys from the United States and its allies have told rebels fighting President Bashar Assad, sources who attended the meeting told Reuters on Tuesday.
U.S. forces in the region are "ready to go", Defense Secretary Chuck Hagel said, as Washington and its European and Middle Eastern partners honed plans to punish Assad for a major poison gas attack last week that killed hundreds of civilians.
Several sources who attended a meeting in Istanbul on Monday between Syrian opposition leaders and diplomats from Washington and other governments told Reuters that the rebels were told to expect military action and to get ready to negotiate a peace.
"The opposition was told in clear terms that action to deter further use of chemical weapons by the Assad regime could come as early as in the next few days, and that they should still prepare for peace talks at Geneva," one of the sources said.
Ahmad Jarba, president of the Syrian National Coalition, met envoys from 11 states in the Friends of Syria group, including Robert Ford, the U.S. ambassador to Syria, at an Istanbul hotel.
United Nations chemical weapons investigators, who finally crossed the frontline to take samples on Monday, put off a second trip to rebel-held suburbs of Damascus. Washington said it already held Assad responsible for a "moral obscenity" and President Barack Obama would hold him to account for it.
However, with Russian and Chinese opposition complicating efforts to satisfy international law - and Western voters wary of new, far-off wars - Western leaders may not pull the trigger just yet. British Prime Minister David Cameron called parliament back from its summer recess for a session on Syria on Thursday.
He and Obama, as well as French President Francois Hollande, face tough questions about how an intervention, likely to be limited to air strikes, will end - and whether they risk handing power to anti-Western Islamist rebels if Assad is overthrown.
In France, which took a vocal lead in helping Libyan rebels topple Muammar Gaddafi in 2011, Hollande was about to address ambassadors. A French diplomatic source said Paris had no doubt Assad's forces carried out the gas attack and would "not shirk its responsibilities" in responding.
In an indication of support from Arab states that may help Western powers argue the case for war against likely U.N. vetoes from Moscow and Beijing, the Arab League issued a statement holding Assad's government responsible for the chemical attack.
In Saudi Arabia, the rebels' leading regional sponsor, Foreign Minister Saud al-Faisal called for "a decisive and serious stand by the international community to stop the humanitarian tragedy of the Syrian people."
Fears of international conflict hit some financial markets, notably in neighboring Turkey, as well as emerging economies that could be hit hard by a chill in world trade.
U.S. FORCES READY
Asked if U.S. forces were ready to strike Syria just "like that", Hagel told the BBC: "We are ready to go, like that."
"We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take," he said. A senior U.S. official told Reuters that Obama had yet to decide on military action.
Top generals from the United States and European and Middle Eastern allies met in Jordan for what could be a council of war.
Hagel said the United States would have intelligence to present "very shortly" about last week's mass poisoning. But he noted after calls with his British and French counterparts that there was little doubt among U.S. allies that "the most base ... international humanitarian standard was violated".
Turkey, Syria's neighbor and part of the U.S.-led NATO military pact, called it a "crime against humanity" that demanded international reaction.
The Syrian government, which denies using gas or obstructing the U.N. inspectors, said it would press on with its offensive against rebels around the capital.
Foreign Minister Walid al-Moualem said U.S. strikes would help al Qaeda allies and called Western leaders "delusional" if they hoped to help the rebels reach a balance of power in Syria.
In Britain, whose forces have supported the U.S. military in a succession of wars, Cameron called for an appropriate level of retribution for using chemical weapons.
"Our forces are making contingency plans," a spokesman told reporters. London would make a "proportionate response".
On Monday, U.S. Secretary of State John Kerry said: "President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people.
"The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity ... And despite the excuses and equivocations that some have manufactured, it is undeniable."
How an intervention, likely to be limited to air strikes, would affect the course of Syria's two and half year old civil war is far from clear. The conflict is largely at a stalemate.
Turmoil in Egypt, whose 2011 uprising inspired Syrians to rebel, has underlined the unpredictability of revolutions. And the presence of Islamist militants, including allies of al Qaeda in the Syrian rebel ranks, has given Western leaders pause. They have held back so far from helping Assad's opponents to victory.
Russia, a major arms supplier to Assad, has said rebels may have released the gas and warned against attacking Syria. Deputy Foreign Minister Gennady Gatilov criticized Washington for cancelling bilateral talks on Syria that were set for Wednesday.
The Syrian conflict has split the Middle East along sectarian lines. Shi'ite Muslim Iran has supported Assad and his Alawite minority against mainly Sunni rebels, some of them Islamists, who have backing from Gulf Arab states.
In Tehran, a foreign ministry spokesman said: "We want to strongly warn against any military attack in Syria. There will definitely be perilous consequences for the region."
Syrian foreign minister Moualem, who insisted the government was trying to help the U.N. inspection team, told a news conference in Damascus that Syria would hit back if attacked.
"We have means of defending ourselves, and we will surprise them with these if necessary," he said. "We will defend ourselves. We will not hesitate to use any means available."
Assad's forces made little or no response to three attacks by Israeli aircraft earlier this year which Israeli officials said disrupted arms flowing from Iran to Lebanon's Hezbollah.
China, which has joined Moscow in vetoing measures against Assad in the U.N. Security Council, is also leary of Western use of force to interfere in other countries' affairs. Beijing's official news agency ran a commentary on Tuesday recalling that the United States invaded Iraq in 2003 on the grounds that it possessed banned weapons, which were never found.
The continued presence of United Nations experts in Damascus may be a factor holding back international military action.
A U.N. statement said the investigators had put off a second visit to the affected areas until Wednesday to prepare better.
Some residents of the capital are getting anxious.
"I've always been a supporter of foreign intervention but now that it seems like a reality, I've been worrying that my family could be hurt or killed," said one woman, Zaina, who opposes Assad. "I'm afraid of a military strike now."
The Syrian opposition proposed 10 targets to the envoys in Istanbul, sources told Reuters. One opposition figure said the rebels were preparing for a possible government collapse:
"The Americans are tying any military action to the chemical weapons issue. But the message is clear; they expect the strike to be strong enough to force Assad to go to Geneva and accept a transitional government with full authority," the source said.
"If the strike ends up to be crippling, and if they hit the symbols of the regime's military power in Damascus it could collapse," the source said.
Some American advocates of the rebel cause questioned the merit of a limited offensive using cruise missiles.
Senator John McCain, who ran against Obama in 2008, said it would be "counterproductive", by leaving Assad in power. He called for providing unlimited weapons supplies to the rebels.
"While we take worse than half measures and the conflict goes on, it becomes more regional, spreading to Lebanon, spreading to Jordan, and of course Syria and Iraq become al Qaeda transit zones as we watch Iraq unravel," he told Reuters.
Opposition activists have said at least 500 people and possibly twice that many were killed by rockets laden with poison, possibly the nerve gas sarin or something similar. If so, it was the worst chemical weapons attack since Saddam Hussein gassed thousands of Iraqi Kurds in 1988.
Israelis have been claiming state-issued gas masks in case Syria responds to a Western attack by firing missiles at Israel, as Saddam did in 1991. Prime Minister Benjamin Netanyahu promised to "respond forcefully" to any attempt to target it.
Additional reporting by Mariam Karouny and William Maclean in Beirut, Phil Stewart in Bandar Seri Begawan, Andrew Osborn in London, John Irish in Paris, Timothy Heritage in Moscow, Ben Blanchard in Beijing, Seda Sezer and Daren Butler in Istanbul, Yeganeh Torbati in Dubai and Lesley Wroughton, Steve Holland and Paul Eckert in Washington; Writing by Alastair Macdonald; Editing by Peter Graff