AMMAN/BEIRUT (Reuters) - The United States and its allies were gearing up on Tuesday for a probable military strike against Syria that could happen within days as punishment for last week’s chemical weapons attacks blamed on President Bashar al-Assad’s government.

Western powers have told the Syrian opposition to expect military action against Assad’s forces soon, according to sources who attended a meeting between envoys and the Syrian National Coalition in Istanbul.

Amid a quickening drumbeat of preparations, U.S. Defence Secretary Chuck Hagel said American forces in the region were “ready to go” if President Barack Obama gives the order, as intelligence agencies assembled what is sure to be final confirmation of the Syrian government’s culpability for Wednesday’s poison gas attack near Damascus.

White House spokesman Jay Carney said it would “fanciful” to think that anyone other than Syrian President Bashar al-Assad’s forces was responsible for the chemical attack, which killed hundreds of people as they slept.

Obama has yet to make a final decision on the U.S. response, Carney said, but left little doubt that it would involve military action. He insisted, however, that Washington was not intent on “regime change,” signaling that any military strikes would be limited and not meant to topple Assad.

The British military was also drafting plans. Prime Minister David Cameron, anxious, like Obama, not to emulate the entanglements in wars in Afghanistan and Iraqi that beset their predecessors, said any strikes would be “specific” so as not to drag the allies deeper into the Syrian civil war now in its third year.

Cameron recalled parliament for a debate on Syria on Thursday.

U. N. chemical weapons investigators put off until Wednesday a second trip to the rebel-held suburbs of Damascus, where activists say hundreds of civilians died a week ago.

While evidence of chemical warfare could bolster an argument for intervention at the United Nations in the face of likely Russian and Chinese opposition, 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 at an Istanbul hotel, including the U.S. ambassador to Syria, Robert Ford. 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 missile or air strikes. There is little public support in Western countries for troops to invade Syria.

“We have moved assets in place to be able to fulfill and comply with whatever option the president wishes to take,” Hagel said.

The precise timing of possible military action remained unclear, but it is certain to wait for an official U.S. intelligence report blaming Assad’s government for the chemical attack. The findings, considered merely a formality at this point, will be released this week, U.S. officials said.

Under growing pressure for U.S. action on Syria Obama is expected to keep close watch on the situation there, but he will go ahead with a speech on Wednesday at Washington’s Lincoln Memorial marking the 50th anniversary of slain civil rights leader Martin Luther King Jr.’s “I have a dream speech.”

MOOD IN DAMASCUS

Syria’s government, backed by Iran, denies gassing its own people and has vowed to defend itself, but residents of Damascus are growing 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 a woman named 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.”

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 of the kind that gave legal backing to NATO air strikes that helped Libyan rebels unseat Muammar Gaddafi two years ago. However, Western officials say they do want to act within international law.

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Moscow and Beijing accuse Western powers of using human rights complaints, such as in Libya, to meddle in sovereign states’ affairs. White House spokesman 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. Carney said it was “profoundly in the interests of the United States” to respond to the chemical weapons attack.

In Britain, 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 using the chemical weapons, citing a 2005 U.N. provision for international action to protect civilians from their own governments.

Similar arguments were used by NATO to bomb Serbia, a Russian ally, after the killing of civilians in Kosovo.

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In an indication of support from Arab states that may help Western powers argue the case for an attack against likely U.N. vetoes from Moscow and Beijing, the Arab League issued a statement holding Assad’s government responsible for the chemical attack.

Fears of another international conflict in the Middle East affected financial markets. Oil prices hit a six-month high and stocks fell around the world, notably in Turkey, as well as in emerging economies that would suffer from a chill in trade.

The government of Turkey, a NATO member, called for action against Assad for what it called a “crime against humanity.”

TOUGH CHOICES

Obama, Cameron and Hollande face questions at home about how a military intervention would 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, where the 2011 uprising inspired Syrians to rebel, has underlined the unpredictability of revolutions. 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 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.

Opposition activists have said at least 500 people, and possibly twice that many, were killed by rockets carrying 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.