Snipers have opened fire on a convoy of United Nations inspectors heading to the sites of suspected chemical weapons attacks in Syria's capital, Damascus.
The UN did not say who fired the shots, but the first car in the convoy was hit and the team returned to a checkpoint. The experts say they will continue the inquiry
The Syrian government and the rebels had agreed to a ceasefire to allow the inspectors to collect evidence.
The ceasefire was announced shortly after a senior United States official told reporters there was "very little doubt" Syrian forces used chemical weapons in the attack.
As many as 350 people are reported to have died in the 21 August attack, which rebels and opposition activists accuse forces supporting President Bashar al-Assad of carrying out.
Mr Assad dismissed the accusations as "an insult to common sense" and state television says rebels were responsible.
However, the US official says if Syria had nothing to hide, it would have granted inspectors immediate access and the decision has come too late to be credible.
There is also concern that evidence would have been significantly corrupted as a result of what the United States says has been persistent shelling in the area of the attack.
British foreign secretary William Hague has also warned that UN inspectors may find the evidence is "compromised".
It is the world's worst apparent chemical weapons attack in 25 years, Reuters reports.
It came a year after US President Barack Obama declared the use of chemical weapons to be a "red line" that would require a firm response.
Meanwhile, Mr Assad has warned the US against military intervention in Syria, saying it would end in failure.
"If someone is dreaming of making Syria a puppet of the West, then this will not happen," he told the Russian newspaper Izvestiya.
'Barbaric' attack
Syrian New Zealander Husam Al-Diery, who has family living just outside Damascus, has described the attack as barbaric.
He says while he's not surprised to hear the regime might have been behind the attack, he is surprised at the scale of it.
"It occurred while they were all asleep as well ... people didn't even have time to react, to get away, there was no warning," he says.
Mr Al-Diery is in the third year of his medical degree and says as soon as he's qualified will travel to Syria to help in any way he can.
Ali Akil, whose extended family lives in Aleppo, has helped set up a group called Syrian Solidarity to raise awareness of the plight of ordinary Syrians.
He says he's shocked by the images of bodies piled high and frustrated at the lack of response from the rest of the world.
