* Envoys tell rebels West will strike "in days" -sources
* Obama not seeking "regime change"
* New UN tests put off; US already blames Assad for gas
* Syrian govt says would hit back, warns against attack
* Russia opposes strikes, China recalls Iraq errors
By Khaled Yacoub Oweis and William Maclean
AMMAN/BEIRUT, Aug 27 (Reuters) - Western forces could attack Syria within days, the United States and its allies have told rebels fighting President Bashar al-Assad, opening up new risks in a war that is spreading hatreds across the Middle East.
Participants at a meeting in Istanbul told Reuters that U.S. and other diplomats warned Syrian opposition leaders on Monday to expect action that would punish Assad for poison gas attacks - and to be ready to negotiate if his government sues for peace.
The United States said its forces in the region were "ready to go", but the White House insisted President Barack Obama was still considering various options, not just military force, and was not intent on bringing about "regime change" in Damascus.
British Prime Minister David Cameron, anxious like Obama not to emulate the Afghan and Iraqi entanglements that beset their predecessors, said any strikes would be "specific", a penalty for the use of chemical weapons, and would not drag the allies deeper into a Syrian civil war now well into its third year.
He recalled parliament for a debate on Syria on Thursday.
United Nations chemical weapons investigators, who finally crossed the frontline to take samples on Monday, put off until Wednesday a second trip to the rebel-held suburbs of Damascus where activists say hundreds of civilians died a week ago.
But while U.N. evidence of chemical warfare could bolster a Western argument for intervention in the face of likely Russian and Chinese opposition at the United Nations, Western leaders - and the Arab League - have already declared Assad guilty.
Ahmad Jarba, president of the rebel Syrian National Coalition, met envoys from 11 countries, including Robert Ford, the U.S. ambassador to Syria, at an Istanbul hotel. The rebel leaders proposed targets for cruise missiles and bombing.
One participant said: "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."
Planning appears to focus on air strikes. There is little public support in Western countries for troops to invade Syria.
U.S. Defense Secretary Chuck Hagel said: "We have moved assets in place to be able to fulfil and comply with whatever option the president wishes to take ... We are ready to go."
The Syrian government, backed by regional power Iran, denies gassing its own people and said it would defend itself.
"NO REGIME CHANGE"
Russia, Assad's main arms supplier, opposes military action and has suggested that rebel forces may have released the poison gas. China's state news agency recalled how flawed intelligence was used to justify the U.S.-led invasion of Iraq in 2003.
Firm opposition from permanent members of the U.N. Security Council all but rules out a U.N. mandate for war of the kind that gave legal backing to NATO air strikes that helped Libyan rebels unseat Muammar Gaddafi two years ago. But Western officials say they do want to act within international law.
Moscow and Beijing accuse Western powers of using human rights complaints, such as in Libya, to meddle in sovereign states' affairs. White House spokesman Jay Carney insisted: "The options that we are considering are not about regime change.
"They are about responding to a clear violation of an international standard that prohibits the use of chemical weapons." Although Obama has long said Assad should step down, he is unwilling to commit to making that happen by force.
Cameron told reporters: "This is not about getting involved in a Middle Eastern war or changing our stance in Syria or going further into that conflict. It's about chemical weapons.
"Their use is wrong, and the world shouldn't stand idly by."
In France, which played a major role in Libya, President Francois Hollande said he was "ready to punish" Assad for the gas attack, citing a 2005 U.N. provision for international action to protect civilians from their own government. Similar arguments were used by NATO to bomb Russian ally Serbia after the killing of civilians in Serbia's then province of Kosovo.
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".
Fears of international conflict in the Middle East affected financial markets. Oil prices hit a six-month high and stocks fell around the world, notably in neighbouring Turkey, as well as emerging economies that would suffer from a chill in trade.
The Turkish government, a NATO member, called for action against Assad for what it called a "crime against humanity".
Obama, Cameron and Hollande face tough questions at home about how an intervention will end, whether they risk bolstering Assad if he rides out the assault and whether they risk handing power to anti-Western Islamist rebels if Assad were overthrown.
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.
Syrian Foreign Minister Walid al-Moualem said U.S. strikes would help al Qaeda and called Western leaders "delusional" if they hoped to help the rebels reach a balance of power in Syria.
"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 that Israeli officials said disrupted arms flowing from Iran to Lebanon's Hezbollah.
The continued presence of United Nations experts in Damascus may be a factor holding back international military action. The experts came under fire in government-held territory on Monday before reaching the rebel lines. They interviewed and took samples from survivors, though much evidence may have decayed.
A U.N. statement said the investigators had put off a second visit to the affected areas until Wednesday to prepare better.
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.
Western governments were besieged by conflicting advice.
Tony Blair, who sent British troops into Afghanistan and Iraq alongside Americans led by George W. Bush, urged today's leaders to stop their "hand-wringing" and act. Failure to intervene, he said, would leave Syria "mired in carnage", a safer haven for Islamist militants than Afghanistan once was.
But General David Richards, until July the head of Britain's armed forces, said "pin-prick" cruise missile strikes could aggravate rather than resolve the conflict, while more sweeping intervention was not on the cards: "The scale of involvement to make a decisive difference in Syria would be so huge that it is something that we, at the moment, cannot sensibly contemplate."
Residents of the Syrian 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 big fear is that they'll make the same mistakes they made in Libya and Iraq," said Ziyad, a man in his 50s. "They'll hit civilian targets, and then they'll cry that it was by mistake, but we'll get killed in the thousands." (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 and Will Waterman)