President Barack Obama told Americans a military strike against Syria is in their interest, although there were signs on Thursday that any action will be delayed at least several days while the case is laid out to U.S. and British lawmakers.
Senior Obama administration officials are expected to brief congressional leaders on Thursday, with lawmakers complaining they have not been properly consulted about plans to respond to what Washington says was the gassing of civilians.
The United Nations said chemical weapons inspectors investigating the attacks will leave Syria on Saturday and then report on their findings to Secretary-General Ban Ki-moon.
In Britain - the closest U.S. ally and a key player in any air assault on Syria - parliament cut short its summer break for a debate on Thursday on Syria likely to reveal deep misgivings stemming from the 2003 invasion of Iraq.
Britain said on Wednesday it would not act until it receives the findings of the U.N. inspectors, after which parliament will be given a chance for a decisive vote.
Increasing expectations of a delay ended a three-day sell off on world share markets on Thursday, but investors were still on edge over future turmoil in the Middle East.
Obama sought to win over a war-weary American public by saying intervention in Syria, where more than 100,000 people have been killed in two and a half years of civil war, would serve U.S. national security interests.
"If we are saying in a clear and decisive but very limited way, we send a shot across the bow saying, 'Stop doing this,' this can have a positive impact on our national security over the long term," he told "PBS Newshour" in a televised interview.
While saying he had not yet made a decision on military action, Obama left little doubt the choice was not whether but when to punish Syria for the gas attacks, which killed hundreds of people in a rebel-held suburb of Damascus.
"We have concluded that the Syrian government in fact carried these out," he said on Wednesday evening.
Syria denies blame for the gas attacks and says they were perpetrated by rebels. Washington and its allies say the denial is not credible.
The United Nations weapons inspectors were already in Syria before last week's attack and have been carrying out on-site investigations. Secretary-General Ban has urged countries to let the team get on with their work.
Britain, which had earlier declined to say whether it would wait for the inspectors to report before launching military action, changed its stance on Wednesday, saying the U.N. Security Council should first see the findings.
Assad's ally Moscow is seen as certain to veto any call for intervention in the U.N. Security Council. A report from Moscow that Russia plans to send two warships to the eastern Mediterranean underscored the complications surrounding even a limited military strike.
Western leaders are expected in Russia next Thursday for a meeting of the Group of 20 big economies, an event that could influence the timing of any strikes. Host President Vladimir Putin has made clear his view that Western leaders are using human rights as a pretext to impose their will on other sovereign states.
Some financial industry analysts now say leaders may wait until after the summit before acting.
"A delay in a probable US military strike against Syria - with or without France and the UK - now looks likely until, we think, after the 5/6 September G20 summit," Nomura Global Markets research said in a note.
A spokesman for the main Syrian opposition umbrella group, the Syrian National Coalition, said the opposition was confident Western leaders were prepared to act.
The spokesman said SNC leader Ahmed Jarba and French President Francois Hollande had discussed a two-wave intervention to first target installations used to launch chemical weapons and then hit other government bases in Syria.
"We are very happy. France and its partners are quite decided to punish the Syrian regime," SNC envoy Monzer Makhous told Reuters after the talks. "Then there will be military aid to help the opposition to change the balance of power."
Hollande urged Jarba to create a credible military force in the talks on Thursday, highlighting Western concern that the mainstream opposition is unable to control al-Qaeda linked militias on the ground in Syria.
In Damascus, residents and opposition forces said President Bashar al-Assad's forces appeared to have evacuated most personnel from army and security command headquarters in the center in preparation for Western military action.
Arguing for measured intervention after long resisting deeper involvement in Syria, Obama insisted that while Assad's government must be punished, he intended to avoid repeating U.S. errors from the Iraq war.
"I have no interest in any open-ended conflict in Syria, but 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," Obama said.
Despite opinion polls showing most Americans oppose deeper involvement in the Syrian conflict, Obama has been under pressure to enforce a "red line" against chemical weapons use, which he declared just over a year ago.
The likeliest option, U.S. officials say, would be to launch cruise missiles from U.S. ships in the Mediterranean in a campaign that would last days.
Obama cited chemical weapons dangers to U.S. Middle Eastern allies Israel, Turkey and Jordan and U.S. bases in the region, and said America's national interests could be at risk if Syrian chemical arms fell into the wrong hands.
COMPLAINTS FROM LAWMAKERS
Although decisive action against Syria is strongly backed by many in the U.S. Congress, there have been growing calls for Obama to seek congressional authorization before ordering the use of force, something he is considered unlikely to do. Wrangling over the issue could complicate any attack timetable.
In Damascus on Wednesday, people left homes close to potential targets as U.S. officials sketched out plans for multi-national air strikes.
Assad said Syria would "defend itself" in the face of any aggression, without giving details.
Syrian officials say the West is playing into the hands of its al Qaeda enemies. The presence of Islamist militants among the rebels has deterred Western powers from arming Assad's foes. But the West says it must now act to stop the use of poison gas.
Diplomats from Russia, China, Britain, France and the United States met at the United Nations on Wednesday to discuss a British resolution for "necessary measures" to protect Syrian civilians. Moscow said the measure was premature.
The United States and its allies say a U.N. veto will not stop them. Western diplomats called the proposed resolution a maneuver to isolate Moscow and rally a coalition behind air strikes.