Involved are government forces, anti-government rebels and Islamic State rebels.
In retaliation for the alleged chemical attack, the US, France and the United Kingdom launched missile strikes against Syrian government facilities, where chemical weapons were supposedly being stored and manufactured.
Hours after reaching Douma, the OPCW in a statement said its team had visited one of the sites of the alleged attack to collect samples for analysis.
The problem is, though, that back on April 1, a week before the alleged attack, the Associated Press was reporting that rebels in Douma were beginning to evacuate the area they had held for years, under a safe passage agreement with the Syrian government, according to which buses would be able to remove them to safety in the north of the country.
On April 13, the United States, Britain and France fired 105 missiles at three Syrian government sites in retaliation.
More than 40 people are said to have died on 7 April.
In a release, OPCW said the samples collected will be transported to its lab in Rijswijk, Netherlands, and then dispatched to the OPCW's designated labs for further analysis.
Assad and his main ally, Russia, have denied responsibility for the attack.
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In a statement Saturday, ministry spokeswoman Maria Zakharova said the team had left for the town and decried the delay as "unacceptable".
Russian Federation has exercised its veto to stifle all U.N. Security Council resolutions sponsored by the United States and European nations to call for sanctions and other measures toward the Assad administration over its suspected use of chemical weapons in recent years.
Other information and "materials" have also been collected by the inspectors, it said.
The committee has been negotiating a deal to spare the city a military assault by the Syrian army and its allies who encircle it.
Even the Foreign Ministry of Russia said Saturday that Russian and Allied forces experienced guaranteed safe passage for the O.P.C.W. team and implied that any flaws inside their trip had been caused by these. It is not mandated to conclude which side in the conflict used them.
The alleged chemical weapons attack reportedly killed at least 60 people and wounded more than 1,000 others in the town, located in Eastern Ghouta on the outskirts of the capital Damascus.