The United States made clear on Friday 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.
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," President Barack Obama told reporters at the White House.
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 White House will 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.
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 August 21 attack, left Syria on Saturday.
The inspectors crossed the land border into neighbouring Lebanon on Saturday, after leaving their Damascus hotel earlier in the day and arrived at Beirut International Airport.
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.
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.
Obama said chemical weapons attacks such as last week's threatened U.S. national security interests as well as U.S. allies such as Israel, Turkey and Jordan.
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.
Assad's government has accused rebels of perpetrating the attacks in order to provoke intervention.
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 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.
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.
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.
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.
Get all the latest news and videos in your inbox. Register FREE