WASHINGTON (Reuters) - The Obama administration made a forceful case on Friday for limited U.S. military action against Syria, releasing evidence the Syrian government had used chemical weapons against civilians several times during the past year and saying the "indiscriminate, inconceivable horror" of a deadly attack last week could not go unpunished.
In separate statements, President Barack Obama and Secretary of State John Kerry harshly condemned the Syrian government but said any military response by the United States would be measured to avoid open-ended commitments - a nod by the White House to most Americans' reluctance to engage in another war.
The White House released a four-page, unclassified intelligence assessment that said an August 21 chemical weapons attack outside Damascus killed at least 1,429 civilians, including 426 children. It was the largest of multiple attacks against the Syrian opposition in the last year, the report said.
In making their case for a U.S. response though not establishing a timetable, Obama and Kerry cast the situation as a matter of national security in controlling chemical weapons, and as a moral imperative for the United States to stand up to rogue nations.
"We cannot accept a world where women and children and innocent civilians are gassed on a terrible scale," Obama said at the White House, calling the attacks a violation of well-established international norms that threatened U.S. allies in the region such as Israel and Jordan.
Kerry called Syrian President Bashar al-Assad "a thug and a murderer," and said that not responding to the attacks would encourage other nations - such as Iran and North Korea - to similarly test international laws.
"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 appearance at the State Department.
"So 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 gonna do about it?" Kerry said.
The release of the intelligence report appeared designed to bolster the administration's case for a military response, as Obama comes under increasing pressure to win support for action from a war-weary public, sceptical lawmakers and reluctant allies.
The report said the rockets that carried chemical weapons in the attack last week were launched from Syrian government-controlled areas and landed in neighbourhoods that are contested or controlled by rebels.
The report said that intercepted communications indicated that Syrian government personnel were told to prepare for the attack by putting on gas masks and taking other precautions normally associated with chemical weapons.
Obama said he had not made a final decision on a response in Syria. Neither he nor Kerry offered any specifics about a possible U.S. response and did not lay out a timetable for action, but they emphasized that any action would be limited.
"It will not involve any boots on the ground. It will not be open-ended. And it will not assume responsibility for a civil war that is already well underway," Kerry said.
The comments by Obama and Kerry came after a meeting of Obama's national security team on Friday morning that included Kerry and National Security Adviser Susan Rice to discuss Syria.
Obama administration officials have said the president was willing to go it alone, if necessary, after the British parliament voted late Thursday against a military strike intended to punish the Syrian government.
Kerry said there was international support for a response - France said on Friday it backed action in Syria. He suggested that the U.S. reaction to the Syrian attack would affect America's standing in world affairs.
"If we choose to live in the world where a thug and a murderer like Bashar al-Assad can gas thousand's of his own people with impunity, even after the United States and our allies said no, and then the world does nothing about it, there will be no end to the test of our resolve and the dangers that will flow," Kerry said.
Defence officials say there has been active debate within the administration about striking the right balance between a limited cruise missile attack aimed at delivering a message about chemical weapons, and a broader attack that could be seen as a strong intervention into Syria's 2 1/2-year-long civil war.
Military and civilian officials have expressed the need for caution to avert a cascading military conflict that could have repercussions throughout the region.
Any military strike appears to be delayed at least until U.N. investigators report back after leaving Syria, and some prominent lawmakers suggested on Thursday after a briefing from the administration that the White House should slow down the rush to action.
The timing is also complicated by Obama's departure for Sweden and a G20 summit in Russia on Tuesday. He was not expected to order the strikes while in Sweden or Russia.
Members of Congress, who do not return to Washington for another week, have demanded that they be consulted about a possible strike. Obama's national security team was to hold at least two more briefings on Syria via conference call for members of Congress on Friday, congressional aides told Reuters.
Polls continue to show the public is largely opposed to U.S. military action.
A Reuters/Ipsos poll released on Friday indicated that most Americans still do not want to intervene, although support for such action has increased since the alleged gas attack near Damascus last week.
Some 53 percent of those surveyed this week said the United States should stay out of the Syria conflict, down from 60 percent last week. Only 20 percent of respondents said the United States should take action, up from 9 percent last week.
After Thursday's briefing, some lawmakers said they were still not convinced of the need for a U.S. response, and others questioned whether the Pentagon could afford to attack Syria after spending cuts imposed on the federal government earlier this year.
"I'm opposed to military intervention. We don't have the resources," Oklahoma Senator James Inhofe, the top Republican on the Senate Armed Services Committee, told MSNBC on Friday.
Rhode Island Democrat Jack Reed, a member of the Senate Armed Services Committee, said on MSNBC that the administration should try to avoid unilateral action. He hoped the release of the U.N. investigators' report would help build public support.
"I think the findings will be very helpful because they'll give an international perspective, one that is factually based, they've been on the ground. They've looked at the evidence. That would help immensely," Reed said.
Former President George W. Bush, who launched the invasion of Iraq in 2003, citing as justification that nation's alleged possession of weapons of mass destruction, told Fox News's "Fox and Friends" program that Obama faced a tough decision.
"I was not a fan of Mr. Assad. He's an ally of Iran and he's made mischief," Bush told Fox. "Putting our military in harm's way is the toughest decision a president will make."
Additional reporting by Susan Heavey, Mark Hosenball, Roberta Rampton and Patricia Zengerle; Writing by John Whitesides; Editing by David Lindsey, Karey Van Hall and Philip Barbara