"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. (Agencies)
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 US Navy destroyers already stationed in the eastern Mediterranean, was uncertain, but it had been considered unlikely before UN weapons experts probing evidence from the August 21 attack, left Syria on Saturday.
The inspectors left their Damascus hotel early on Saturday, a source said, with their convoy of vehicles heading onto a highway that leads to neighbouring Lebanon.
A sixth US warship is now operating in the eastern Mediterranean, defense 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 US attack despite Thursday's British parliamentary ‘no’ vote that left Washington without one of its closest allies.
Kerry said that "if a thug and a murderer like Bashar al-Assad can gas thousands of his own people with impunity," it would set a bad example for others, such as Iran, Hezbollah and North Korea.
Obama said chemical weapons attacks such as last week's threatened US national security interests as well as US allies such as Israel, Turkey and Jordan.
"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 UN 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 that UN Secretary-General Ban Ki-moon told Security Council members it may be two weeks before final results of the tests are ready.
"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.