US says Russia, Syria trying to 'sanitise' Douma chemical attack site
"That is of great concern to us," she said.
A team from the Organisation for the Prohibition of Chemical Weapons arrived in Syria just hours after unprecedented US-led strikes on regime targets on Saturday, launched in response to the alleged gas attack.
A visit by international inspectors to the location of the suspected gas attack in Douma was delayed on Wednesday after gunfire at the site during a visit by UN security personnel.
The visit has been put on hold and the UN says the security team must return to check out security measures before the fact-finding mission can go to Douma.
Britain's UN ambassador said on Thursday the Russians and Syrians must uphold the promises they made to let chemical weapons inspectors visit the site of a suspected poisonous gas attack in the Damascus suburb of Douma - and "it should happen expeditiously."
Karen Pierce told The Associated Press that "it's incumbent upon them more than ever to allow the team in, to escort it, to make sure it's safe, and to make sure it can do its work."
She said it's "very important" that the World Health Organization, a UN body, "has found at least 500 cases of people reporting the symptoms of a toxic gas attack."
The British official said "it has to be a possibility" that the Russians have tampered with evidence of the attack, but added that "we don't yet have the facts."