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By Lesley Wroughton and Paul Eckert
WASHINGTON, Aug 26 (Reuters) - U.S. Secretary of State John Kerry said on Monday that the evidence of a massive deadly chemical attack last week was "undeniable" and accused the Syrian government of trying to cover it up, signaling the United States was edging closer to a possible military response.
In the most forceful U.S. reaction yet after Wednesday's suspected 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 on Monday from victims of last week's apparent chemical 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 accused the government of President Bashar al-Assad of acting like it 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.
The information so far, including videos and accounts from the ground, indicate that chemical weapons were used in Syria, he said.
"It is undeniable," he said, adding that it was the Syrian government that maintained custody of chemical weapons and had the rockets capable of delivering them.
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.
There were mounting signs that the United States and Western allies were laying the groundwork for some kind of military response to 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 the conflict 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.
But with his international credibility seen increasingly on the line, Obama could opt for limited measures such as a missile strike to punish Assad and seek to deter further chemical attacks, without dragging Washington deeper into the fight. (Additional reporting by Paul Eckert, Susan Cornwell, Tabassum Zakaria; Writing by Matt Spetalnick; Editing by Sandra Maler and David Storey)