Experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) will travel to the Syrian city of Douma on Wednesday, Russian Defence Ministry official Igor Kirillov said at a news conference in The Hague on Monday.

The United States have accused Russia of blocking international inspectors from reaching the site of a suspected poison gas attack in Douma and said Russians or Syrians may have tampered with evidence on the ground.

Syria and Russia deny unleashing poison gas on April 7 during their offensive on Douma, which ended with the recapture of the town that had been the last rebel stronghold near the capital, Damascus.

Relief organisations say dozens of men, women and children were killed. Footage of young victims foaming at the mouth and weeping in agony has thrust Syria's civil war - in which half a million people have been killed in the past seven years - to the forefront of world concern again.

Inspectors from the Organisation for the Prohibition of Chemical Weapons (OPCW) travelled to Syria last week to inspect the site, but have yet to gain access to Douma, which is now under government control after the rebels withdrew.

‘It is our understanding the Russians may have visited the attack site,’ U.S. Ambassador Kenneth Ward said at an OPCW meeting 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 Moscow had interfered with any evidence. ‘I can guarantee that Russia has not tampered with the site,’ he told the BBC.

Increasing regional jitters, Syrian anti-aircraft defences shot down missiles fired at the air base of Shayrat in Homs province late on Monday and at another base northeast of the capital, Damascus, Syrian state television and pro-Iranian Hezbollah media said.

The Pentagon said there was no U.S. military activity in that area ‘at this time.’

An Israeli military spokesman said: ‘We don't comment on such reports’.

The UN Security Council is due to be briefed on Tuesday, at the request of Russia, on the situation in Syria's northern city of Raqqa, where Islamic State was defeated last year by U.S.-backed forces, and the Rubkan camp for displaced Syrians near the country's border with Jordan and Iraq.