The UN investigating team will report back by the end of the week on suspected chemical weapons attacks in Syria, Secretary-General Ban Ki-moon says, as the US and its allies hold back from immediate punitive military action against Damascus.
US President Barack Obama said he has yet to sign off on a plan to strike Damascus over the horrific attacks that activists say killed hundreds of people and threatened to draw the West into the brutal 29-month conflict in Syria.
A Western air bombardment had appeared imminent earlier this week, but US allies were increasingly reluctant to act before hearing the results of a UN probe into the alleged poisonous gas attacks last Wednesday.
As the UN chemical weapons experts headed out to one of the attack sites near Damascus for the third day of inspections on Thursday, Ban said the team would leave Syria by Saturday and report to him immediately.
He appealed to international powers to work together to head off military action in Syria, where the UN says around 100,000 people have been killed and several million made homeless since the conflict erupted in March 2011.
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"Diplomacy should be given a chance ... peace (should) be given a chance," Ban said, but added: "The use of chemical weapons by anyone, for any reason, under any circumstances, is a crime against humanity and that must be held accountable for."
Key Damascus allies Russia and Iran again on Thursday warned against any Western intervention in the civil war, saying it could set off a wider regional conflict.
Syria's nervous neighbours have already stepped up their preparations for conflict, with Israel authorising a partial call-up of arm reservists while Turkey put its forces on heightened "vigilance".
Russia was reportedly sending warships to the Mediterranean, while Britain said it was sending fighter jets to the strategic island of Cyprus.
Syrian President Bashar al-Assad vowed his country would emerge the victor in any confrontation with the US.
"Since the start of the crisis ... we have waited for our true enemy to reveal itself," Al-Akhbar newspaper quoted Assad as telling Syrian officials.
"It's a historic confrontation from which we will emerge victorious," he added.
In Damascus, soldiers were being pulled back from their command posts and tougher security controls were in place at roadblocks and hospitals as the country braced for a possible US allied bombardment.
International pressure for action mounted after grisly pictures emerged after Wednesday's attacks showing dead children who appeared to have been gassed to death.
But some commentators have warned that any US attack would effectively see Washington fighting on the side of al-Qaeda, whose fighters have joined rebels in the battle to oust Assad.
In London, MPs were set to vote on a response to the attacks, but the government conceded on Wednesday there should be no British military strike until the weapons inspectors had reported back.
Obama, who has warned that the use of chemical weapons in Syria would cross a US "red line", said on Wednesday that Washington had definitively concluded that the Assad regime was to blame.
A senior White House official told AFP that the administration will brief senior US legislators on Thursday about classified intelligence on the chemical attack.
Asked how close he was to ordering a US strike, expected to start with cruise missile raids, Obama told PBS NewsHour: "I have not made a decision."
He said US action would be designed to send a "shot across the bow" to convince Syria it had "better not do it again".
The US leader and Nobel Peace laureate, who wants to seal a legacy of ending foreign wars, not getting into new ones, argued that it was vital to send a clear message not just to Syria, but around the world.
"We do have to make sure that when countries break international norms on weapons like chemical weapons that could threaten us, that they are held accountable."
Washington had bluntly signalled that a UN Security Council resolution that could have given a legal basis for an assault was going nowhere, owing to Russian opposition.
"We cannot be held up in responding by Russia's continued intransigence at the United Nations, and quite frankly the situation is so serious that it demands a response," State Department deputy spokeswoman Marie Harf said.
British Prime Minister David Cameron was forced by a parliamentary revolt to pledge he would not order military action until the report by UN inspectors has been published.
White House officials would not immediately say whether Washington would wait for Britain before launching any military action.
"The region is like a gunpowder depot," Iran's supreme leader Ayatollah Ali Khamenei warned in a condemnation of the West's reported military plans.
Chinese media warned the West against attacking.
In an editorial headed "No excuse for strikes", the state-run China Daily said the US and its Western allies were "acting as judge, jury and executioner".
