Syria and Russian Federation 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.
It raises concerns that Russian Federation may have tampered with evidence at the site where dozens of civilians were killed.
Following weekend missile strikes on Syria by the US, France and Britain, Russia traded accusations with Western nations on Monday, dismissing as "a blatant lie" accusations that Moscow was hindering the investigation in Douma.
Analysts close to the Kremlin say that Moscow will continue to back the increasingly successful military campaign to restore all of Syria under the control by Bashar al-Assad's government, as well as to pursue the peace process, excluding the U.S. and its allies, which it initiated with Iran and Turkey previous year.
OPCW Director-General Ahmet Uzumcu said Syrian and Russian officials cited "pending security issues" in keeping its inspectors from reaching Douma.
Other people, speaking to different media outlets including the Associated Press and CBS News, said there was a chemical attack and that they experienced the symptoms of such an attack. Several said a unusual smell started spreading and people screamed, "It's chlorine!"
"The UNDSS (the UN security team) will continue to work with the Syrian National Authority, the local councils in Douma, and the Russian Military Police to review the security situation".
On Tuesday, the United Nations security team was escorted by Russian military police to two sites in Douma, a town near Damascus that was retaken last week following a Russian-backed Syrian offensive.
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Independent experts from the Organization for the Prohibition of Chemical Weapons are still waiting to visit the town, 11 days after the alleged attack took place.
In a statement, the French foreign ministry said it was "essential that Syria give full, immediate and unimpeded access to all the OPCW's requests, whether relating to sites to visit, people to interview or documents to consult".
"Cleaning up a site after a chemical attack to the point where no traces of the agent or of characteristic degradation products can be found is hard, in particular when the impact site was a building".
Tensions between the US and Russian Federation have escalated in recent days over the conflict in Syria.
Syria's ally Russian Federation was given prior notice and the missiles struck mostly empty buildings, in what many analysts saw as a hollow move that allowed all sides to save face.
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. Many were found with foam around their mouths, an indicator for suffocation.
He and two other residents accused the rebel Army of Islam of carrying out the attack. Separately, the AP spoke to a medic who was among those who later were evacuated to northern Syria. He said he tried to enter the shelter but was overcome by a strong smell of chlorine and his comrades pulled him out.
The Council of the European Union also met on Monday, and issued a statement afterwards strongly condemning "the continued and repeated use of chemical weapons by the regime in Syria, including the latest attack on Douma, which is a grave breach of worldwide law and an affront to human decency".