Unidentified snipers have opened fire on a convoy of UN experts investigating suspected chemical weapons attacks in Syria's capital, the UN has said.
One car was shot at "multiple times", forcing the convoy to turn back.
Syrian state media blamed opposition "terrorists" for the attack, though the claim could not be verified.
The UN team later resumed its mission, entering the western district of Muadhamiya to gather evidence, before returning to central Damascus.
Hundreds died in alleged attacks on Wednesday in five districts near Damascus.
The US said there was little doubt that Syrian forces used chemical weapons in the attacks, which reportedly killed more than 300 people in rebel-held areas.
Syrian President Bashar al-Assad dismissed the accusation as "an insult to common sense" and warned the US against military intervention.
"If someone is dreaming of making Syria a puppet of the West, then this will not happen," he told the Russian newspaper Izvestiya.
The 20-member UN inspection team has been in Syria since 18 August to look into three earlier suspected chemical attacks. They were given permission on Sunday to examine the Damascus locations.
The experts intend to take soil, blood, urine and tissue samples for laboratory testing but they are unlikely to apportion blame for any of the attacks.
Video footage posted online appears to show UN inspectors in Muadhamiya taking samples and talking to residents.
They went to a Red Crescent centre and spoke to doctors, opposition activists said.
On the video, which the BBC has not been able to fully authenticate, one resident is heard telling an inspector of heavy raids on the district, with "over 600 canister strikes...12 tanks, 100 soldiers".
Shortly after setting out from their hotel in Damascus, the inspectors' cars came under fire "multiple times by unidentified snipers", according to a statement from the UN.
The team returned safely back to the government checkpoint before setting out again.
The convoy was "deliberately targeted" and it seemed someone was trying to intimidate the team, the UN Secretary General's spokesman, Farhan Haq, told the BBC.
The inspectors have now returned to their hotel and are expected to resume their work on Tuesday.
A year ago, US President Barack Obama said the use of chemical weapons by the Syrian government would be "a red line" that could trigger US military action.
Washington has recently bolstered its naval presence in the eastern Mediterranean, and military leaders from the US, UK and their allies are meeting in Jordan.
But the UN Security Council remains divided, with China and Russia appearing unlikely to drop their objection to stricter sanctions on Mr Assad's government.
Chinese Foreign Minister Wang Yi said on Monday that diplomats should be cautious in dealing with the chemical weapons issue, and Moscow warned Western nations not to prejudge the outcome of the inspections.
Western politicians have begun to suggest taking action outside of the UN system.
UK Foreign Secretary William Hague told the BBC that action could be taken without UN approval if there was "great humanitarian need" in Syria.
His French counterpart Laurent Fabius suggested the UN Security Council could be bypassed "in certain circumstances".
But in his latest comments on the crisis, Russian Foreign Minister Sergei Lavrov said any intervention in Syria without a UN mandate would be a "grave violation of international law".
The West, he told a news conference in Moscow, had not been able to come up with any proof of chemical weapons use while "saying at the same time that the red line has been crossed and there can be no delay".
Western officials were unimpressed with Syria's decision to allow in the UN experts.
Mr Hague said evidence could have been tampered with, degraded or destroyed in the five days since the attack.
A senior White House official, quoted by AP news agency, dismissed the visit as "too late to be credible".
Medecins Sans Frontieres (MSF) said on Saturday that three hospitals it supports in the Damascus area had treated about 3,600 patients with "neurotoxic symptoms" on Wednesday morning, of whom 355 had died.
While MSF said it could not "scientifically confirm" the use of chemical weapons, staff at the hospitals described a large number of patients arriving in the space of less than three hours with symptoms including convulsions, pinpoint pupils and breathing problems.
Syria's security forces are widely believed to possess large undeclared stockpiles of mustard gas and sarin nerve agent.
It is one of seven countries that have not joined the 1997 convention banning chemical weapons.
- 01:15: 21 August (10:15 GMT 20 Aug): Facebook pages of Syrian opposition report heavy fighting in rebel-held eastern districts of the Ghouta, the agricultural belt around Damascus
- 02:45: Opposition posts Facebook report of "chemical shelling" in Ein Tarma area of the Ghouta
- 02:47: Second opposition report says chemical weapons used in Zamalka area of the Ghouta
- Unverified video footage shows people being treated on pavements in the dark and in a makeshift hospital
- Reports say chemical weapons were used in Ghouta towns of Irbin, Jobar, Zamalka and Ein Tarma as well as in Muadhamiya to the west, but this is not confirmed
- Syrian government acknowledges military offensive in the Ghouta but denies chemical weapons use