The fact-finding mission from the Organization for the Prohibition of Chemical Weapons is investigating reports that government forces launched an April 7 chemical attack in the final stages of their campaign to retake the town from rebels.
The Group of Seven leading industrialized nations has issued a joint statement endorsing the US -led air strikes in Syria.
Activists on the ground in Syria said the attack killed more than 40 people and injured hundreds more sheltering from bombing in basements beneath the city.
The US, UK and France say that, based on open source information and their own intelligence, they are confident chlorine and possibly a nerve agent were used.
Experts from the worldwide chemical weapons watchdog are in Syria and are now expected to visit that site in the town of Douma Wednesday. It also reported a separate air strike on the Dumayr air base near Damascus.
Deputy Russian foreign minister Sergei Ryabkov blamed the Western air strikes carried out early on Saturday for holding up a mission by the OPCW team to Douma.
In the meantime, the Russian military have found a laboratory operated by militants in central Douma, which the military says was used to produce chemical weapons.
Russian Federation denied the claims, saying parts of Douma still needed to be de-mined and said the watchdog's inspectors would enter on Wednesday.
On Monday, Syrian and Russian authorities prevented investigators from the Organization for the Prohibition of Chemical Weapons from going to the scene, the head of the OPCW said, blocking global efforts to establish what happened and who was to blame. She stressed that it is Russian Federation and Syria that bear the responsibility for the security of the OPCW experts. Britain denies the allegation.
Less than two days later, the Army of Islam rebels surrendered the town, which was the last stronghold in the once rebellious eastern Ghouta region at the doorstep of Damascus.
"Not even a day passed after the missile strikes when their organizers started putting forward unusual and political initiatives", Nebenzia said.
The accounts contradict what the Syrian government and Russian Federation have reported: that there was no gas attack in Douma.
He said any use of chemical weapons is in complete disregard of humanity, and is reprehensible and contrary to the provisions of the Chemical Weapons Convention as well as accepted worldwide norms.
Separately, Foreign Minister Sergei Lavrov told the BBC, "I can guarantee that Russian Federation has not tampered with the site".
Jerry Smith, a former chemical weapons inspector in Syria and founder of the RameHead Consulting International security firm, warned that a substance like chlorine could "disappear in a couple of hours".