US President Barack Obama said the United States, which has five cruise-missile equipped destroyers in the region, was in the planning process for a "limited, narrow" military action that would not involve boots on the ground or be open-ended.
In a sign the United States may be preparing to act, US Secretary of State John Kerry spoke on Friday to the foreign ministers of key European and Gulf allies, as well as the head of the Arab League, a senior State Department official said.
A witness said the team of UN experts arrived at Beirut International Airport on Saturday, after crossing the land border from Syria into Lebanon by foot earlier in the day.
The 20-member team, including experts from the Organisation for the Prohibition of Chemical Weapons, have been into the rebel-held areas in the Ghouta suburb of Damascus three times, taking blood and tissue samples from victims. They also took samples of soil, clothing and rocket fragments.
They will be sent to laboratories in Europe, most likely Sweden or Finland, for analysis. The UN experts have already been testing for sarin, mustard gas or other toxic agents.
The analysis should establish if a chemical attack took place but not who was responsible for the Aug. 21 attack on a Damascus suburb.
Final results might not be ready for two weeks, UN Secretary-General Ban Ki-moon told Security Council members, according to diplomats.
The United States released its own unclassified intelligence report on the attack, which Kerry said killed 1,429 Syrian civilians and was clearly the work of Assad's forces.
"If we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity" it would set a bad example for others, such as Iran, Hezbollah and North Korea, Kerry said.
Syria blames rebels fighting to topple Assad for the attack . Its main ally Russia, which has repeatedly used its UN Security Council veto to block forceful action against the Syrian leader, says any attack on Syria would be illegal and only inflame the civil war there.
Syria's Foreign Ministry repeated its denial that the government had used chemical weapons against its own people. Kerry's accusations were a "desperate attempt" to justify a military strike. "What he said was lies," the ministry said.
A military strike on Syria by the United States had been considered unlikely while UN experts were still in Syria but even after their departure, its exact timing remained uncertain.
The White House was to brief Republican senators on Syria in a conference call on Saturday at the request of Senate Minority Leader Mitch McConnell, a spokesman for the senator said.
In comments to told reporters at the White House on Friday, Obama set no timetable for action.
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," he said.
In Syria itself, activists and residents in and around Damascus readied themselves for a strike.
A man named Youssef carried a small plastic bag bulging with documents. "Do I put them in my parents' home My in-laws At work I don't know which area is safer, I don't know where to hide them," he told a friend.
"That's my marriage certificate, my passport, my home ownership deed, my college degree, and all my wife's documents too. We can't figure out where to put them for safekeeping."
Doctors in the outskirts of the capital said they were training up teams and trying to secure shipments sent in by aid groups of atropine and oxygen, which are needed for treating chemical weapons victims.
"We worry about another chemical weapons attack should foreign powers carry out the strike, as some kind of revenge, or who knows what could happen," said a doctor in the rebel-held suburb of Arbin, called Abu Akram.
Rebels said they were also preparing for a strike.
"We're all waiting for the unknown now, waiting to hear about the timing," said one rebel source speaking by Skype, who asked not to be named.
"The rebels are trying to come up with some kind of a plan to capitalize on any strike but it's difficult when we don't know when, where or how."