UN inspectors are due to visit the site of a suspected chemical weapons attack that took place on Wednesday near the Syrian capital Damascus.
The Syrian government and the rebels agreed to a ceasefire to allow the inspectors to collect evidence safely.
However, Western governments criticised Syria for taking too long to allow in the UN team.
Syrian President Bashar al-Assad warned the US against military intervention, 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.
The US says there is little doubt Syrian forces used chemical weapons in the attack, which reportedly killed more than 300 people.
Mr Assad dismissed the accusations as “an insult to common sense”.
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 bolstered its naval presence in the eastern Mediterranean, and military leaders from the US, UK and their allies are meeting in Jordan.
Syria’s stockpiles
The suspected chemical strike occurred in the Ghouta area of eastern Damascus, which is under the control of rebels fighting to depose Mr Assad.
Government artillery and warplanes have pounded the area for days.
Hans Blix: “It’s important that [the inspectors] can go to any place they want to see”
The UN inspectors will have to pass through both government-held and rebel-controlled areas.
Monday’s visit was negotiated by UN disarmament chief Angela Kane, a UN statement said. She arrived in Damascus on Saturday.
The 20-member UN inspection team has been in Syria since 18 August to look into three earlier suspected chemical attacks.
Correspondents say their mandate was to determine whether such weapons were used, not who was responsible for unleashing them, and there is no indication that the mission’s brief has changed.
The BBC’s Yolande Knell in Beirut says the work of the UN team is likely to involve taking soil, blood, urine and tissue samples for laboratory testing.
Syria is 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.
‘Neurotoxin symptoms’
Even as Monday’s visit was announced, Western officials cautioned that the delay in agreeing to it may have compromised any findings.
UK Foreign Secretary William Hague warned that 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”, saying Washington had “very little doubt” that President Assad’s forces used such weapons.
Russia, a key ally of Syria, welcomed the decision to allow the inspectors in but warned the West against pre-empting the results.
Medecins Sans Frontieres (MSF) said on Saturday that three hospitals it supports in the Damascus area had treated about 3,600 patients with “neurotoxin symptoms” on Wednesday morning, of whom 355 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.
On Saturday, UK Prime Minister David Cameron and President Obama threatened a “serious response” if it emerged that Syrian forces had used chemical weapons.
US Defence Secretary Chuck Hagel has said the American military, which is repositioning naval forces in the Mediterranean, was ready to act if called upon.
- 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
BBC
