But UN spokesman Stephane Dujarric said the United Nations had "provided the necessary clearances for the OPCW team to go about its work in Douma".
The weekend's airstrikes by the US, France and United Kingdom have increased worldwide tension, as the US and Russian Federation exchanged threats of retaliation. "And on Wednesday is when we plan the arrival of the OPCW experts", a senior Russian official said at a press conference in the Russian embassy in The Hague, explaining the roads were still being cleared of mines.
Syrian air defenses responded firmly to the Western powers' attacks, shooting down most of the missiles fired at the country.
He denied that the government of President Bashar al-Assad was responsible.
"The team has not yet deployed to Douma", Ahmet Uzumcu, director-general of the Organization for the Prohibition of Chemical Weapons (OPCW), said on April 16.
The April 7 strike on Douma, in which most experts say chlorine as well as an agent such as sarin were used, killed at least 40 people, according to local medics.
"Investigators will look for evidence that shows whether the incident site has been tampered with", Trapp said, adding that they will also have to find ways of authenticating evidence that it presented to by third parties.More news: Man charged in attempted robbery
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U.S. media quoted a USA representative to the OPCW as saying that Russian Federation may have tampered with evidence.
OPCW officials have carried out similar chemical checks in Syria.
The government's capture of Douma, the last town held by the rebels in eastern Ghouta, marks a major victory for Assad.
Regime forces have since entered Douma and declared the entire Eastern Ghouta region around it fully retaken, ending a five-year siege and reclaiming an opposition bastion on the edge of the capital.
But they have made none of that evidence public even after they, along with Britain, bombarded sites they said were linked to Syria's chemical weapons program. "Suffocation cases arrived as a result of the debris, dust, fire and smoke". Photographs and video from Douma documented victims suffering from symptoms consistent with exposure to toxic chemicals, such as asphyxiation and foaming at the mouth, the officials said.
The report showed people waving Syrian flags, alongside those of Iran and Russia- the main allies of Syrian President Bashar Assad during years of the country's conflict - in the face of what many called "limited" or even "failed" strikes created to punish Assad for his alleged use of chemical weapons.