Also today, Russian President Vladimir Putin urged Obama not to rush into any decision on striking Syria, but to consider whether strikes would help end the violence and be worth the likely civilian casualties.
Today's statements were the first from Putin since the suspected chemical weapons attack Aug. 21.
Putin advised Obama to reflect on the results of U.S. military intervention in Afghanistan and Iraq before deciding to unleash air strikes against Syrian President Bashar Assad's regime.
The Russian leader implied the chemical attack came from among the rebels, saying he was convinced it was a provocation carried out by those who wanted to draw in the United States.
If the Americans have evidence to the contrary they should present it to the United Nations inspectors and the U.N. Security Council, he said.
“If there is evidence it should be presented,” Putin said. “If it is not presented, that means it does not exist.”
After four days of on-site inspections, the U.N. team wrapped up its investigation Friday into the suspected chemical attack on rebel-held suburbs of Damascus.
The experts took with them blood and urine samples from victims as well as soil samples from the affected areas for examination in laboratories in Europe.
The United Nations has said it will try to expedite its final report, and U.N. disarmament chief Angela Kane was to brief Secretary-General Ban Ki-moon later today on the investigation.
The inspectors' departure brings the looming confrontation between the U.S. and Assad's regime one step closer to coming to a head.
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 inject a dangerous new dynamic into a Syrian civil war that has already killed more than 100,000 people, forced nearly 2 million to flee the country and inflamed sectarian tensions across the Middle East.
While Obama has long been wary of U.S. military involvement in the conflict, the administration Friday forcefully made its case for action against Syria in the wake of last week's events outside Damascus.
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.
With France as his only major public ally, Obama said 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 TV 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 stocked up on food and other necessities in anticipation of strikes, with no evident sign of panic.
Syrian state TV this 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 station's morning talk shows were dominated by discussions about a potential U.S. military strike.
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.
The 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.”
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.”
Friday's events produced sharply differing responses from members of Congress, even within the same party.
Sens. John McCain of Arizona and Lindsey Graham of South Carolina, both Republicans, said Obama needed to go further than he seems to be planning, saying the goal should be “to shift the balance of power on the battlefield against Assad and his forces.”
But a spokesman for Republican House Speaker John Boehner, Brendan Buck, said if the president believes in a military response to Syria, “it is his responsibility to explain to Congress and the American people the objectives, strategy, and legal basis for any potential action.”
Obama has long 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.