When they arrive at the site on Wednesday, it will be 11 days since the attack.
Russian Federation and the Syrian government deny responsibility for the alleged attack.
"Last night, a false alarm that Syrian air space had been penetrated triggered the blowing of air defence sirens and the firing of several missiles", a military source said.
On Monday, OPCW Director-General Ahmet Uzumcu said Syrian and Russian authorities had blocked its inspectors from going to Douma and instead offered them 22 people to interview as witnesses.
France's foreign ministry on Tuesday said it was very likely that proof has disappeared from the site of a suspected poison gas attack in Syria, adding it was essential that global inspectors be given full access to the site.
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British Prime Minister Theresa May says Syria and Russian Federation - whose forces now control the town east of Damascus - are trying to cover up evidence.
Al-Ikhbariya TV says the fighters from the Army of Islam rebel group and their families have begun evacuating the town of Dumayr, bound for opposition-held areas in the north as part of the agreement.
Meanwhile early on Sunday morning, President Trump tweeted, "The Syrian raid was so perfectly carried out, with such precision, that the only way the Fake News Media could demean was by my use of the term 'Mission Accomplished.' I knew they would seize on this but felt it is such a great Military term, it should be brought back".
But in the aftermath of the coordinated airstrikes, which hit a chemical weapons research center near Damascus and two storage facilities, USA and British officials stressed that they had been extremely careful to avoid any civilian casualties.
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 OPCW chief said the two suspected attack sites were under the control of the Russian military police. A unusual smell lingered, nine days after the attack. Many were found with foam around their mouths, an indicator for suffocation. As they spoke, government troops were nearby but out of earshot. They are still waiting to visit nearby Douma, 11 days after the alleged attack took place.
Former Douma residents now in northern Syria told The National anyone still in Douma was likely unable to speak freely for fear of retribution from the government.
While the United States went after ISIS and "actually achieved lasting gains for the Syrian people, the Assad regime was busy bombarding civilians in places like Aleppo, Idlib and Eastern Ghouta", Currie said. He said he tried to enter the shelter but was overcome by a strong smell of chlorine and his comrades pulled him out.