By Lesley Wroughton and Matt Spetalnick
WASHINGTON (Reuters) - Secretary of State John Kerry laid the groundwork on Monday for possible military action against the Syrian government over a chemical weapons attack, implicating President Bashar al-Assad's forces in a "moral obscenity."
In the most forceful U.S. reaction yet to last week's gas attack outside Damascus, Kerry said President Barack Obama "believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people."
Kerry spoke after U.N. chemical weapons experts interviewed and took blood samples from victims of the attack in a rebel-held suburb of Syria's capital, after the inspectors themselves survived sniper fire that hit their convoy.
"What we saw in Syria last week should shock the conscience of the world," Kerry told reporters. "Let me be clear: The indiscriminate slaughter of civilians, the killing of women and children and innocent bystanders by chemical weapons is a moral obscenity."
Kerry's tough language marked an increased effort by the administration not only to point the finger at Assad's government but to prepare the war-weary American public for a potential military response.
He accused the Syrian rulers of acting like they had something to hide by blocking the U.N. inspectors' visit to the scene for days and shelling the area.
"Our sense of basic humanity is offended not only by this cowardly crime, but also by the cynical attempt to cover it up," Kerry said.
Information gathered so far, including videos and accounts from the ground, indicate that the use of chemical weapons in Syria was "undeniable," Kerry said, adding that it was the Syrian government that maintained custody of the weapons and had the rockets capable of delivering them.
A STEP CLOSER TO MILITARY RESPONSE
There were mounting signs that the United States and Western allies were edging closer to a military response over the incident, which took place a year after Obama declared the use of chemical weapons a "red line" that would require strong action.
Obama, who withdrew troops from Iraq and is winding down U.S. involvement in Afghanistan, has been reluctant to intervene in two and a half years of civil war in Syria.
A Reuters/Ipsos poll published on Saturday showed about 60 percent of Americans opposed U.S. military intervention, while only 9 percent thought Obama should act.
However, with his international credibility seen increasingly on the line, Obama could opt for limited measures such as cruise missile strikes to punish Assad and seek to deter further chemical attacks, without dragging Washington deeper into the war.
The United States has started a naval buildup in the region to be ready for Obama's decision, and an administration official said Obama's aides were continuing a series of high-level meetings to determine a course of action.
Kerry stopped short of explicitly blaming the Syrian government for the gas attack but strongly implied that no one else could have been behind it and said the United States had "additional information it would provide in the days ahead.
White House spokesman Jay Carney said there was "very little doubt" that the Syrian government was to blame but that Obama had not yet decided how to respond.
The administration has not set a timeline for responding but officials are preparing options for Obama with a sense of urgency, the State Department said.
Kerry said the administration, which has reached out to foreign allies to coordinate a response, was "actively consulting" members of Congress, though some lawmakers said they had not been fully informed. Republicans in particular have long pressed Obama to act more forcefully against Assad.
A spokesman for John Boehner, the Republican Speaker of the House of Representatives, said Boehner had not been consulted before he had "preliminary communication" with the White House about the situation in Syria on Monday afternoon. Boehner told the White House it must present "clearly defined objectives."
A U.S. security source said that as of Monday, Washington and its allies still did not have conclusive scientific evidence that the attack involved chemical weapons, and that such proof could take days or weeks to gather.
But sources said while the evidence may be "circumstantial," U.S. intelligence has "high confidence" that chemical weapons were used by Assad's forces.
"Intelligence agencies are still analyzing data and information related to the attack and are preparing a final assessment for the president," an intelligence official said.
(Additional reporting by Paul Eckert, Susan Cornwell, Tabassum Zakaria, Mark Hosenball, Steve Holland, Jeff Mason, Mark Felsenthal and Patricia Zengerle; Writing by Matt Spetalnick; Editing by Alistair Bell and Christopher Wilson)