By Steve Holland and Catherine Bremer
WASHINGTON/PARIS (Reuters) - 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 can not 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.
Secretary of State John Kerry said 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.
The timing of the attack, most likely with cruise missiles from U.S. Navy destroyers already stationed in the eastern Mediterranean, was uncertain, but it was unlikely to come before U.N. weapons experts leave Syria on Saturday.
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 U.S. national security interests as well as U.S. 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 August 21 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 U.N. 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 timing of any strikes may be complicated by Obama's departure late on Tuesday for Sweden and a G20 summit in Russia.
Kerry and Obama were speaking the day after British Prime Minister David Cameron failed to win parliamentary backing for military action in Syria.
Finance minister George Osborne, one of Cameron's closest allies, accepted that the vote had raised questions about Britain's future relations with its allies.
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. However, the defeat of the government motion authorizing 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.
Western diplomats have been seeking a vote in the 15-member Council on a draft measure, which would authorize "all necessary force" in response to the suspected gas attack, to isolate Moscow and show that other nations back military action.
But China said there should be no rush to force a council decision on Syria until the U.N. inspectors complete their work.
The United Nations said its experts had completed the collection of samples and evidence from last week's attack. U.N. spokesman Martin Nasik 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.
(Additional reporting by Matt Spetalnick, Andrea Shalal-Esa, Patricia Zengerle, Thomas Ferraro and Jeff Mason in Washington, Erika Solomon and Oliver Holmes in Beirut, Khaled Yacoub Oweis in Amman, Sarah Marsh in Berlin, Timothy Heritage in Moscow, Phil Stewart in Manila, Louis Charbonneau and Michelle Nichols at the United Nations, Lidia Kelly in Moscow, Ben Blanchard and Michael Martina in Beijing, John Irish in Paris and Andrew Osborn, Marie-Louise Gumuchian and Peter Apps in London; Writing by Alistair Lyon and Claudia Parsons; editing by David Storey and Jim Loney) Interactive timeline: http://link.reuters.com/rut37s