UN inspectors visited Monday the scene of a suspected chemical weapons attack near the Syrian capital after braving sniper fire, as the West weighed possible military action against the regime.
The arms experts spoke to victims of the August 21 attack, UN chief Ban Ki-moon said from Seoul.
The secretary general also said the United Nations had made "a strong complaint" to both the Syrian government and rebels over the sniper attack, for which the rival sides traded blame.
Despite the "very dangerous circumstances" faced by the team, he said the investigators "visited two hospitals, they interviewed witnesses, survivors and doctors, they also collected some samples."
The hospitals are in the Moadamiyet al-Sham suburb of Damascus.
A sniper attacked the UN convoy when it made an initial attempt to enter Ghouta, east of Damascus, the other venue of the alleged toxic gas attacks in which hundreds of people were reportedly killed last Wednesday.
Opposition rebels and the government accuse each other of having used the banned weaponry.
The opposition says more than 1,300 people died when toxic gases were unleashed on Eastern Ghouta and Moadamiyet al-Sham. Doctors Without Borders said 355 people died of "neurotoxic" symptoms in the affected areas.
In Moadamiyet al-Sham, the UN team took samples and interviewed medical staff during a visit which lasted almost three hours before returning to their hotel in Damascus.
The government approved the UN inspection on Sunday but US officials said it was too little, too late, arguing that persistent shelling there in recent days had "corrupted" the site.
The inspection came as the West appeared to be moving closer to launching a military response over the alleged attack near Damascus that shocked the world after grisly pictures emerged of dead children apparently gassed to death.
A defiant President Bashar al-Assad, meanwhile, declared that any strike by the US and its allies would be doomed to failure and key ally Moscow warned of dangerous consequences for the entire region.
A Downing Street spokesman said British Prime Minister David Cameron was cutting short his holiday to deal with the crisis and would meet with top cabinet ministers.
Russian news agencies later reported that President Vladimir Putin and Cameron discussed the crisis by telephone.
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.
Washington and its allies have pointed the finger of blame at Assad's regime for the alleged poison gas attack, the latest atrocity in a conflict that has claimed the lives of more than 100,000 people since March 2011.
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A US official told reporters travelling with US Defence Secretary Chuck Hagel that Washington was convinced the Syrian regime was behind the attack.
"Our confidence is growing that this was in fact an episode involving the use of chemical weapons by the Syrian regime," the official said in Jakarta, speaking on condition of anonymity.
Assad, in an interview with a Russian newspaper published Monday, denied such accusations as an "insult to common sense" and said any military action was doomed to fail.
"The United States faces failure just like in all the previous wars they waged," he said.
A senior Syrian security official told AFP the regime was ready to face "all scenarios."
"Western threats of strikes against Syria are part of the psychological and political pressure against Syria, but in any case we are ready to face all scenarios," the official said.
Russian Foreign Minister Sergei Lavrov warned his US counterpart John Kerry of the "extremely dangerous consequences of a possible new military intervention for the whole Middle East and North Africa region."
Lavrov told reporters the West was currently moving towards "a very dangerous path, a very slippery path."
"Using force without the approval of the UN Security Council is a very grave violation of international law," Lavrov said.
The international community has long been divided over how to respond to the conflict, with Russia and China repeatedly blocking UN Security Council resolutions.
US President Barack Obama has been loath to order US military action to protect civilians in Syria, fearing being drawn into a vicious civil war, soon after he extracted US troops from Iraq.
But revulsion over video footage and gruesome photographs of dead children blanketing the world's media have seen mounting pressure on the international community.
France said the West would decide in the coming days on a response.
"The only option that I do not envisage is to do nothing," Foreign Minister Laurent Fabius said on French radio.
US officials said that Obama, who had said a year ago that the use of chemical weapons by Assad's forces was a "red line" that could trigger Western intervention, would make an "informed decision" about how to respond.
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.
A US defence official said the navy would expand its presence in the Mediterranean with a fourth American warship armed with cruise missiles.