Gathering environmental evidence will be especially complicated, Famini said.

Both agents believed to be used in the attack degrade over time, making tests for the actual agent difficult, he said.

The team had been waiting for a week to enter Douma, just a 10-kilometre drive north-east from where they were staying in the capital, because of what Russian and Syrian authorities said were security issues.

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The city was the recent focal point of a large-scale government campaign aimed at routing armed rebel factions from Ghouta, the suburban areas ringing the capital.

During the final phase of that assault, opposition activists and medical workers contend troops loyal to Syrian President Bashar Assad bombed two areas of Douma with chemical-filled barrel bombs, metal barrels packed with explosives and detritus that the government has dropped often on rebel-held areas.

Medics said hundreds of patients exhibited symptoms including skin discoloration, burning eyes and foaming at the mouth, indications of exposure to what they said was chlorine gas, a chemical poison, and sarin, a nerve agent.

Both Syria and Russia, Assad's top international backer, dismissed the claims and invited the OPCW to investigate.

The rebels surrendered after the attack, and Damascus had declared the area "clear of terrorists," its usual term for the rebels, six days ago. Military police, numerous officials and even foreign journalists visited the area without incident.

Yet, a United Nations advance security team sent this week to the two areas attacked in Douma withdrew after it came under small arms fire and an explosive was detonated, the OPCW's director-general, Ahmet Uzumcu, said in a statement Wednesday.

The OPCW's visit opens another chapter in the confrontation between Russia and Western nations long opposed to Assad's rule.

US, French and British officials have accused Russia of tampering with the site and obstructing the work of the OPCW, even as they insist they already have sufficient proof to hold Assad responsible for the attack.

"We appeal to Western colleagues to refrain from actions obstructing efforts to establish the truth regarding the provocation in Eastern Ghouta," Zakharova said. Moscow expected the OPCW to conduct "an impartial investigation" and submit "an unbiased report as soon as possible," Zakharova said.

Los Angeles Times