Washington warned Syria it would face action over the "moral obscenity" of a gruesome chemical weapons attack, as UN inspectors braved sniper fire to gather evidence about the incident.
A very sudden drumbeat toward some kind of US and/or allied retaliation against Syria seemed to be getting louder.
The US cancelled a meeting with Damascus ally Russia on the Syrian conflict that had been scheduled for this week in The Hague, the State Department said.
In Asia, stocks were down and oil prices were up early Tuesday -- both shifts blamed on fears of yet another escalation in the brutal 29-month-old Syrian war, this time via a direct American intervention that President Barack Obama has steadfastly tried to avoid. As recently as Friday, he seemed to rule it out as messy and hard to get out of.
Speaking amid reports that Washington and its allies are preparing to launch a punitive cruise missile strike on Syrian targets, US Secretary of State John Kerry accused President Bashar al-Assad's regime of engaging in a cover-up.
"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 declared in a televised statement.
"By any standard it is inexcusable, and despite the excuses and equivocations that some have manufactured, it is undeniable."
Kerry said Washington would provide more evidence of who was behind the attack, and that Obama was determined the guilty would face consequences.
"We have additional information about this attack, and that information is being compiled and reviewed together with our partners, and we will provide that information in the days ahead," he warned.
"Make no mistake. President Obama believes there must be accountability for those who would use the world's most heinous weapons against the world's most vulnerable people. Nothing today is more serious."
Kerry was speaking as UN inspectors met survivors of last week's attack, which the independent medical agency Doctors Without Borders has said left at least 355 people dead from "neurotoxic symptoms".
The UN convoy came under sniper fire as it tried to approach the Damascus suburb where the attack was reported, but the team nevertheless managed to visit victims receiving treatment in two nearby hospitals.
"It was a very productive day," UN spokesman Farhan Haq told reporters, adding that the team, led by Swedish expert Ake Sellstrom, is "already gathering valuable evidence".
UN leader Ban Ki-moon said that despite the "very dangerous circumstances" the investigators "visited two hospitals, they interviewed witnesses, survivors and doctors. They also collected some samples".
The UN team was in a buffer zone between government and opposition-held areas when it came under attack.
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Ban said the United Nations had made a "strong complaint" to the Syrian government and opposition forces. The rebels and Assad's government traded blame for the sniper assault just as they did the chemical attack.
The United States accused Syrian government forces of resuming their shelling of the attack site soon after the UN team departed in a bid to destroy evidence.
President Vladimir Putin of Russia -- a staunch Assad ally that provides the regime with diplomatic cover by blocking UN Security Council action -- meanwhile remained unimpressed by the mounting evidence of an atrocity.
Putin on Monday told British Prime Minister David Cameron there was no proof Damascus had used chemical weapons, according to Cameron's office.
Cameron cut short his holiday on Monday to return to London to plan a response. Britain, along with France, has been in the forefront of demands for tougher action against Assad's regime.
Senior military officers from Western and Muslim countries started gathering in Jordan Monday to discuss the regional impact of the war in Syria, Jordanian officials said.
US army chief General Martin Dempsey will take part, as would chiefs of staff from Saudi Arabia, Qatar, Turkey, Britain, France, Germany, Italy and Canada, said an official, cited by state news agency Petra.
A senior Israeli delegation meanwhile visited the White House for high-level talks on the Syrian crisis and the showdown over Iran's controversial nuclear program.
The Syrian opposition says more than 1,300 people died when toxic gases were unleashed on Eastern Ghouta and Moadamiyet al-Sham, two neighborhoods on the outskirts of Damascus.
Syria approved the UN inspection on Sunday, but US officials said it was too little, too late, arguing that persistent shelling had "corrupted" the site.
The inspection came as the West appeared to be moving closer to launching a military response, after officials confirmed the US Navy has four warships armed with cruise missiles on standby in the eastern Mediterranean.
With China and Moscow expected to boycott any resolution backing a military strike, British Foreign Secretary William Hague said the West could act even without full UN Security Council backing.
There is also precedent for Obama to act militarily without US congressional backing, despite a law technically requiring it.
The alleged poison gas attack is only the latest atrocity in a conflict that has claimed more than 100,000 lives since March 2011.
Assad, in an interview with a Russian newspaper published Monday, denied accusations his government was behind the attack, calling the charges an "insult to common sense".
"The United States faces failure just like in all the previous wars they waged," he added.
Russian Foreign Minister Sergei Lavrov meanwhile warned of the "extremely dangerous consequences of a possible new military intervention" and said intervening without a UN Security Council resolution would be illegal.
Experts believe the most likely US action would see sea-launched cruise missiles target Syrian military installations and artillery batteries deemed complicit in the chemical weapons attack.