US Secretary of State John Kerry. (AFP)
In the most forceful US reaction yet to last week's gas attack outside Damascus, Kerry on Monday 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 UN 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 Syria's President Bashar al-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 UN 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.
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 US 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% of Americans opposed US military intervention, while only 9% 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 build-up 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 spokesperson 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.
Timeline for responding
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 spokesperson 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 US 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," US intelligence has "high confidence" that chemical weapons were used by Assad's forces.
"Intelligence agencies are still analysing data and information related to the attack and are preparing a final assessment for the president," an intelligence official said.
'Perilous consequences'
Meanwhile, Iran warned on Tuesday against foreign military intervention in Syria, saying the resulting conflict would engulf the region.
Iran, which is supporting Assad against rebels seeking to overthrow him, has said rebels were behind the suspected attack and said on Tuesday the West was using it as a pretext to intervene in Syria.
"We want to strongly warn against any military attack in Syria. There will definitely be perilous consequences for the region," Iranian foreign ministry spokesperson Abbas Araqchi told a news conference. "These complications and consequences will not be restricted to Syria. It will engulf the whole region."
Shi'ite Iran is Syria's closest ally and has accused an alliance of militant Sunni Islamists, Israel and western powers of trying to use the conflict to take over the region.
As well as backing Assad, Iran also supports the Lebanese Shi'ite militant group Hezbollah which has sent fighters into Syria to help the government there. Tension is also rising between Hezbollah and Israel over cross-border rocket attacks. – Reuters