In Moscow, Russian President Vladimir Putin urged Obama not to rush into a decision. The Russian leader said he was convinced the attack was a provocation carried out by those who want to draw the U.S. into the conflict, but that Washington should show any evidence to the contrary to the United Nations inspectors and the U.N. Security Council.
"If there is evidence it should be presented," Putin said. "If it is not presented, that means it does not exist."
Russia is one of Syrian President Bashar Assad's staunchest allies. Putin's comments were his first on the crisis since the suspected chemical weapons attack on rebel-held suburbs of Damascus on Aug. 21.
The U.N. inspectors spent three days this week touring stricken areas near Damascus and a fourth day interviewing patients at a government-run military hospital. They wrapped up their investigation Friday and left Syria on Saturday, via Lebanon.
Later Saturday, the team was en route to the Dutch city of Rotterdam aboard a German government-chartered plane, the German Foreign Ministry said.
The experts took blood and urine samples from victims as well as soil samples from the affected areas for examination in laboratories in Europe. The U.N. has said it will try to expedite its final report. U.N. disarmament chief Angela Kane is to brief Secretary-General Ban Ki-moon later Saturday on the investigation.
With the inspectors now out of Syria, the looming confrontation between the U.S. and Assad's regime moves one step closer to coming to a head. Most observers viewed U.S. military action as unlikely while the U.N. team was still inside Syria, but the Obama administration has made clear that it is confident in its assessment and could act before the U.N. releases the results of its probe.
Obama has said that if he opts for a military strike, any operation would be limited in scope and only aimed at punishing Assad for his alleged use of chemical weapons.
But U.S. action carries the potential to trigger retaliation by the Syrian regime or its proxies against U.S. allies in the region, such as Jordan, Turkey and Israel. That would be dangerous new turn for the Syrian civil war, which has already killed more than 100,000 people, forced nearly 2 million to flee and inflamed tensions across the Middle East.
While Obama long has been wary of U.S. military involvement in the conflict, the administration on Friday forcefully made its case for action. It accused the Assad regime of carrying out what it says was a chemical attack on Aug. 21 that killed at least 1,429 people - far more than previous estimates - including more than 400 children.
However, the Syrian Observatory for Human Rights, one of the main groups monitoring casualties in Syria, said Saturday it has only been able to confirm 502 deaths, identifying victims by name.
Its list is based on interviews with hospital officials and activists in the affected areas, said Rami Abdel-Rahman, the head of the Observatory.
The Britain-based group, which draws its information from a network of anti-regime activists in Syria, urged the Obama administration to release the information its far higher death toll is based on.
With France as his only major public ally, Obama told reporters he has a strong preference for multilateral action.
"Frankly, part of the challenge we end up with here is a lot of people think something should be done but nobody wants to do it," he said.
The U.S. already has warships in place in the eastern Mediterranean Sea near Syria's coastal waters. The vessels are armed with cruise missiles, long a first-line weapon of choice for presidents because they can strike distant targets without need of air cover or troops on the ground.
The Syrian government dismissed the administration's claims as "flagrant lies" akin to faulty Bush administration assertions before the Iraq invasion that Saddam Hussein had weapons of mass destruction. A Foreign Ministry statement read on state television late Friday said that "under the pretext of protecting the Syrian people, they are making a case for an aggression that will kill hundreds of innocent Syrian civilians."
In Damascus, residents braced themselves in anticipation of strikes.
"We are anticipating it starting tonight, since the inspectors have left, but we don't really know," said Nour, who lives on the outskirts of Damascus.
"Just in case, we stocked up on some water and food. Our building has a basement that we can use as a shelter. The building supervisor started preparing it a couple of days ago, he cleaned it and we put some pillows, blankets, water and a first aid kit with basic medications," said the 23-year-old pharmacy student. She gave only her first name for security reasons.
Syrian state TV on Saturday morning broadcast footage of Syrian soldiers training, fighter jets soaring in the sky and tanks firing at unseen targets, all to the backdrop of martial music. The potential U.S. military strike dominated the station's morning talk shows.
Obama met with his national security aides at the White House on Friday and then with diplomats from Baltic countries, saying he has not yet made a final decision on punitive strikes.
But the administration did nothing to discourage the predictions that action was imminent. That impression was heightened both by sharply worded remarks from Secretary of State John Kerry and the release of an unclassified intelligence assessment that cited "high confidence" that the Syrian government carried out the attack.
In addition to the dead, the assessment reported that about 3,600 patients "displaying symptoms consistent with nerve agent exposure" were seen at Damascus-area hospitals after the attack.
In his remarks, Kerry added that "a senior regime official who knew about the attack confirmed that chemical weapons were used by the regime, reviewed the impact and actually was afraid they would be discovered."
The U.S. assessment did not explain its unexpectedly large count of 1,429 deaths, far in excess of an estimate from Doctors Without Borders from earlier this week that put the toll at 355. Not surprisingly - given the nature of the disclosure - it also did not say expressly how the United States knew what one Syrian official had allegedly said to another.
Mindful of public opinion, Kerry urged Americans to read the four-page assessment for themselves. He referred to Iraq - when Bush administration assurances that weapons of mass destruction were present proved false, and a U.S. invasion led to a long, deadly war. Kerry said this time it will be different, and that "we will not repeat that moment."
Citing an imperative to act, the secretary of state said "it is directly related to our credibility and whether countries still believe the United States when it says something. They are watching to see if Syria can get away with it because then maybe they, too, can put the world at greater risk."
While Obama was having trouble enlisting foreign support, French President Francois Hollande was an exception. The two men spoke by phone, then Hollande issued a statement saying they had "agreed that the international community cannot tolerate the use of chemical weapons, that it must hold the Syrian regime responsible and send a strong message to denounce the use of (such) arms."
Obama long has been wary of U.S. military involvement in Syria's civil war, as he has been with tumultuous events elsewhere during the so-called Arab Spring. In the case of Syria, his reluctance stems in part from recognition that while Assad has ties to Iran and the Lebanese militant group Hezbollah, the rebels seeking to topple him have connections with al-Qaida-linked groups.
Still, Obama declared more than a year ago that the use of chemical weapons would amount to a "red line" that Assad should not cross. Obama approved the shipment of small weapons and ammunition to the Syrian rebels after an earlier reported chemical weapons attack, although there is no sign the equipment has arrived.
Associated Press writers Yasmine Saker and Karin Laub in Beirut, and Geir Mouslon in Berlin contributed to this report.