The United States made clear that it would punish Syrian President Bashar al-Assad for the "brutal and flagrant" chemical weapons attack that it says killed more than 1,400 people in Damascus last week.
He said the United States was still in the planning process for a "limited, narrow" military response that would not involve "boots on the ground" or be open-ended. He set no timetable for action.
In a sign the United States may be preparing to act, a senior State Department official said Secretary of State John Kerry spoke on Friday to the foreign ministers of Britain, Egypt, Germany, the Netherlands, New Zealand, Saudi Arabia and the United Arab Emirates as well as to the secretary general of the Arab League.
The timing of the attack, most likely with cruise missiles from five U.S. Navy destroyers already stationed in the eastern Mediterranean, was uncertain, but it had been considered unlikely before U.N. weapons experts probing evidence from the Aug. 21 attack, left Syria on Saturday.
A sixth U.S. warship is now operating in the eastern Mediterranean, defence officials said late on Friday. One of the officials said the USS San Antonio's passage into the Mediterranean was long-planned, but officials thought it prudent to keep the ship near the destroyers given the current situation.
Kerry said on Friday it was essential not to let Syria get away with the attack, partly as a sign to those who might consider using chemical weapons in the future.
"History would judge us all extraordinarily harshly if we turned a blind eye to a dictator's wanton use of weapons of mass destruction," Kerry said in a televised statement.
Syria's Foreign Ministry repeated the government's denial that it had used chemical weapons and said Kerry's accusations were a "desperate attempt" to justify a military strike. "What he said was lies," the ministry said of Kerry's statement.
The witness saw the team's convoy of vehicles head onto a highway that leads to neighbouring Lebanon.
With France on Friday affirming its support for a military response to punish Assad's government, the statements from Obama and Kerry appeared to harden the resolve for a U.S. attack despite Thursday's British parliamentary "no" vote that left Washington without one of its closest allies.
"So, I have said before, and I meant what I said, that the world has an obligation to make sure that we maintain the norm against the use of chemical weapons," he said.
Kerry laid out a raft of evidence he said showed Assad's forces were behind the attack, and the U.S. government released an unclassified intelligence report at the same time including many of the details. The report said the attack killed 1,429 Syrian civilians, including 426 children.
The intelligence included an intercepted communication by a senior official intimately familiar with the attack as well as other intelligence from people's accounts and intercepted messages, the four-page report said.
"Any action that he (Obama) might decide to take will be (a) limited and tailored response to ensure that a despot's brutal and flagrant use of chemical weapons is held accountable," Kerry said.
Assad's government has accused rebels of perpetrating the attacks in order to provoke intervention.
Syrian state television, which did not carry Kerry's speech live, reported that Kerry said the "first and last" aim of any action the Obama administration will carry out in the Middle East was to "guarantee the security of Israel."
Kerry said the U.N. inspectors' report would only confirm that chemical weapons were used and made clear it would not change much for Washington since "guaranteed Russian obstructionism" would make it impossible for the United Nations to galvanize world action.
"The primary question is really no longer, what do we know? The question is, what are we - we collectively - what are we in the world going to do about it?" Kerry said.
The United Nations said its experts had completed the collection of samples and evidence from last week's attack. U.N. spokesman Martin Nesirky said all the analysis of the samples must be completed before conclusions can be drawn.
Diplomats said U.N. Secretary-General Ban Ki-moon told Security Council members it may be two weeks before final results of the tests are ready.
Kerry and Obama were speaking the day after British Prime Minister David Cameron failed to win parliamentary backing for military action in Syria.
French President Francois Hollande told the daily Le Monde he still supported taking "firm" punitive action over an attack he said had caused "irreparable" harm to the Syrian people.
Hollande is not constrained by the need for parliamentary approval of any move to intervene in Syria and could act before lawmakers debate the issue on Wednesday. An official in his office said the French president spoke to Obama on Friday and "they showed great determination about this crisis."
Britain has traditionally been the United States' most reliable military ally. But the defeat of the government motion authorising a military response in principle underscored misgivings dating from how the country decided to join the U.S.-led invasion of Iraq in 2003.
Russia, Assad's most powerful diplomatic ally, opposes any military intervention in Syria, saying an attack would increase tension and undermine the chances of ending the civil war.
Yuri Ushakov, senior foreign policy adviser to Russian President Vladimir Putin, said the British vote represented majority opinion in Europe.
"People are beginning to understand how dangerous such scenarios are," he told reporters. "Russia is actively working to avert a military scenario in Syria."
Turkish Prime Minister Tayyip Erdogan said any intervention in Syria should be aimed at ending Assad's rule - a goal that Obama has ruled out.
Kerry said the U.S. intelligence community had carefully reviewed and re-reviewed information regarding the attack. "I will tell you it has done so more than mindful of the Iraq experience. We will not repeat that moment."
Kerry said that three days before the attack, the Syrian government's chemical weapons personnel were on the ground in the area, making preparations.
"And we know that the Syrian regime elements were told to prepare for the attack by putting on gas masks and taking precautions associated with chemical weapons."
"We know 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 that they would be discovered," Kerry said.
Assad's forces fired rockets on Friday at a Damascus suburb hit by poison gas last week in another attempt to capture the strategic area, opposition activists said.
Polls show the American public is largely opposed to U.S. military action, and after a Thursday briefing some lawmakers said they were still not convinced of the need for it. Some questioned whether the Pentagon could afford to attack Syria after spending cuts imposed this year.
A new Reuters/Ipsos poll on Friday showed 53 percent of those surveyed this week said the United States should stay out of Syria's civil war, down from 60 percent last week. Twenty percent said the United States should take action, up from 9 percent last week.
Analysts said there was little doubt about the message from Obama's administration. "I would now be shocked if the U.S. government doesn't go to war, even if alone," said Bilal Saab, director and head of research at the Institute for Near East and Gulf Military Analysis. "This is as clear a case for intervention, from the administration's standpoint, as can be."
Some allies, including Italy, have warned that military action without U.N. Security Council authorisation may make matters worse. Russia holds veto power as a permanent Security Council member and has blocked three resolutions meant to press Assad to stop the violence since the revolt began in 2011.