There is a "possibility" that Russia has tampered with evidence of an alleged chemical weapons attack in Douma, the trigger for US, UK and French air strikes in Syria, the British ambassador to the UN said.
Karen Pierce said all the facts had not yet been established after the April 7 attack and said Russia and Syria should allow inspectors to visit the site.
The visit to the area by inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) has been put on hold after a team came under small arms fire and returned to Damascus earlier this week.
Ms Pierce, speaking to the Associated Press, said: "We do look to the Russians and the Syrians to uphold the promises they made to the Security Council last week that the inspectors would be allowed in, that they would be escorted, that they would be safe and that they could have free access."
She said the investigators should be allowed to visit the site "expeditiously", adding "it's incumbent upon them (Russia and Syria) 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."
When asked if she was concerned Russia had tampered with evidence of the attack which is believed to have killed 75, Ms Pierce said: "It has to be a possibility. It has to be, and one could construe... that was responsible for the delay.
"But I stress, we don't yet have the facts, we are waiting for the UN report."
Russia has said chemical weapons were not used in Douma, in an attack which the United States, United Kingdom and France say was carried out by the regime of Bashar Assad.
In response to the attack, four of the UK's Royal Air Force Tornado GR4s joined co-ordinated missile strikes, launching Storm Shadow missiles at a base 15 miles west of Homs.
Speaking last weekend, British Prime Minister Theresa May said there was "no practicable alternative" than to use force to deter the use of chemical weapons by the Syrian regime.
She added: "This is not about intervening in a civil war. It is not about regime change."
Asked about the strikes and whether they "burnished" Mr Assad's image, Ms Pierce said: "I think it's exposed him as a suspected war criminal who gasses and attacks and bombs his own people with illegal weapons.
"I don't think it burnished his image at all."