Western countries accused Moscow on Monday of preventing inspectors from reaching the site of a suspected poison gas attack in Syria and said Russians or Syrians may have tampered with evidence on the ground.
The United States, Britain and France launched air strikes on Saturday against what they described as three Syrian chemical weapons targets in retaliation for a suspected gas attack that killed scores of people in the Damascus suburb of Douma on April 7.
Syria and its ally Russia deny using poison gas during their offensive this month, in which they seized the town that had been the last major rebel stronghold near the capital.
Inspectors from the Organization for the Prohibition of Chemical Weapons (OPCW) went to Syria last week to inspect the Douma site but have yet to gain access to the town, which is now under government control after the rebels withdrew.
"It is our understanding the Russians may have visited the attack site," US Ambassador Kenneth Ward said at a meeting of the OPCW in The Hague on Monday.
"It is our concern that they may have tampered with it with the intent of thwarting the efforts of the OPCW Fact-Finding Mission to conduct an effective investigation," he said. His comments at the closed-door meeting were obtained by Reuters.
Russian Foreign Minister Sergei Lavrov denied that Moscow had interfered with any evidence: "I can guarantee that Russia has not tampered with the site," he told the BBC in an interview.
Earlier, Britain's delegation to the OPCW accused Russia and the Syrian government of preventing the international watchdog's inspectors from reaching Douma.
The inspectors aim to collect samples, interview witnesses and document evidence to determine whether banned toxic munitions were used, although they are not permitted to assign blame for the attack.
"Unfettered access is essential," the British delegation said in a statement. "Russia and Syria must cooperate."
Moscow blamed the delay on the Western air strikes. Kremlin spokesman Dmitry Peskov said the British accusation that Russia was to blame for holding up the inspections was "groundless".
"We called for an objective investigation. This was at the very beginning after this information [of the attack] appeared. Therefore allegations of this towards Russia are groundless," Peskov said.
Witnesses and Western governments say helicopters dropped chemical bombs that killed many children and women hiding in cellars from bombardment. Washington has said it has conclusive evidence the attack used chlorine gas, and suspects sarin nerve agent was also used although this was not confirmed.
Syria agreed to give up its chemical weapons arsenal in 2013 and submit to OPCW inspections to avert US retaliation after a suspected nerve gas attack in Douma killed hundreds of people.
It is barred from having, storing or using nerve agents, and while it is permitted to possess chlorine for civilian uses, is banned from using that chemical as a weapon.
The British envoy to the OPCW said the body had recorded 390 allegations of the use of banned chemicals in Syria since 2014, and that a failure by the OPCW to act risked allowing "further barbaric use of chemical weapons".
US President Donald Trump has said the weekend air strikes accomplished their aim of undermining efforts by the Syrian government to produce and use chemical weapons.
Members of the 41-seat executive council of the OPCW were due to discuss the alleged use of prohibited toxins in Syria, but were not expected to reach any agreement about a response.
The organisation, which needs a two-thirds majority to take decisions, has been undermined by political divisions over the use of chemical weapons by the Syrian government.
A joint United Nations-OPCW mission concluded that troops under President Bashar al-Assad had used chemical weapons several times in recent years, including a sarin attack a year ago in the town of Khan Sheikhoun that killed nearly 100 people.