US President Barack Obama said the United States, which has five cruise-missile equipped destroyers in the region, was planning "limited, narrow" military action to punish Assad for an attack that Washington said killed 1,429 people.
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," Obama said on Friday after Washington unveiled an intelligence assessment concluding Assad's forces were to blame for the attack.
The August 21 attack - the deadliest single incident of the Syrian civil war and the world's worst use of chemical arms since Iraq's Saddam Hussein gassed thousands of Kurds in 1988 -has galvanized a reluctant Washington to use force after two and a half years on the sidelines.
After laying out the case in a televised speech, US secretary of state John Kerry spoke on Friday to the foreign ministers of European and Gulf allies and the head of the Arab League. He and other top administration officials were due to hold a classified briefing for Democratic and Republican senators on Saturday, the White House said.
France is also expected to join any US strikes.
"The chemical massacre in Damascus cannot and must not go unpunished. Otherwise we'd run the risk of an escalation that would trivialise the use of these arms and put other countries at risk," French President Francois Hollande said on Friday.
The team of UN experts drove up to Beirut International Airport on Saturday after crossing the land border into Lebanon by road earlier in the day. No Western intervention had been expected as long as they were still on the ground in Syria.
The 20-member team had arrived in Damascus three days before the Aug. 21 attack to investigate earlier accusations. After days holed up in a hotel, they visited the sites several times, taking blood and tissue samples from victims in rebel-held suburbs of Damascus and from soldiers at a government hospital.
Other UN agencies have also pulled staff from Syria, and countries have warned citizens away from neighbouring Lebanon.
"Most of the mid-level and non-essential staff left on Thursday. The heads of the various agencies have stayed behind, together with a skeleton local staff," a UN source said from Damascus on Saturday.
Washington says it need not wait for the inspectors to report, since it is already certain poison gas was used and convinced Assad's forces were behind it. The inspectors mandate is to determine if chemicals were used, not who used them.
Polls show military intervention is unpopular in the United States, France and other Western countries. Obama acknowledged that Americans were "war weary" after 12 years of conflict in Iraq and Afghanistan, the longest wars in American history.
Such weariness cost Washington the support of its closest ally: Britain has also backed action but was forced to pull out of the coalition after Prime Minister David Cameron unexpectedly lost a vote over it in parliament on Thursday, straining London's "special relationship" with Washington.
Kerry said Washington must act to protect itself and its allies, including Syria's neighbours Turkey, Jordan and Israel, from future use of banned weapons.
"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 embolden others, such as Iran, Hezbollah and North Korea, Kerry said.
Syria and its main ally Russia say rebels carried out the gas attack as a provocation. Moscow has repeatedly used its UN Security Council veto to block action against Syria and says any attack would be illegal and only inflame the civil war there.
"I am convinced that (the chemical attack) is nothing more than a provocation by those who want to drag other countries into the Syrian conflict," President Vladimir Putin said.
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.
Washington says the Syrian denials are not credible, and the rebels would not have been able to launch such an attack.
Syria neighbour Turkey backs the use of force. The Arab League, whose members mainly oppose Assad, has said Syria is to blame for the chemical attack but so far stopped short of explicitly endorsing Western military strikes. Arab League foreign ministers are due to meet in Cairo on Sunday.
Iran, Assad's main ally in the region, has condemned plans for strikes and warned of wider war.
Read more: Obama weighs 'limited, narrow' action against Syria
Residents prepare for strikes
In Syria itself, Damascus residents readied for a strike.
A man named Youssef carried a small plastic bag bulging with personal 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.
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 to treat poison gas victims.
"We worry about another chemical weapons attack should foreign powers carry out the strike, as some kind of revenge," said Abu Akram, a doctor in the rebel-held suburb of Arbin.
Rebels said they were planning to take advantage of a strike to launch an offensive. Qassim Saadeddine, a former Syrian army colonel and spokesman for the rebels' Supreme Military Council, said rebel groups had been sent a military plan of action.
"The hope is to take advantage when some areas are weakened by any strikes. We ordered some groups to prepare in each province, to ready their fighters for when the strike happens," he said by Skype. "They were sent a military plan that includes preparations to attack some of the targets we expect to be hit in foreign strikes, and some others we hope to attack at the same time."
Syria's civil war has killed more than 100,000 people and driven millions from their homes since 2011, when Assad's forces cracked down on street protests and his enemies took up arms.
The war splits the Middle East on its main faultline between Sunni and Shi'ite Muslims and has already spread to neighbouring Iraq and Lebanon, threatening to reignite their own civil wars.
Despite demanding Assad step down, the United States and its Western allies have not provided rebels with arms to unseat him, much less intervened along the lines of NATO air strikes that brought down Libya's Muammar Gaddafi in 2011.
After rebel gains, Syrian government forces returned to the offensive this year with the aid of fighters from Lebanon's Hezbollah Shi'ite militia. Limited strikes of the sort Obama envisions would do little to end the stalemate.
The rebels are mostly majority Sunnis, fighting rule by Assad's Alawite minority sect, an off-shoot of Shi'ite Islam.
They are armed by Qatar and Saudi Arabia and train in Turkey and Jordan, all ruled by Sunnis. Assad is armed by Shi'ite Iran.
Some of the most successful rebel groups are fiercely anti-Western Sunni Islamists allied to al Qaeda, and the West is wary of arming the opposition for fear of weapons reaching them.