The team was earlier not allowed to visit Douma.

The OPCW team will gather soil and other samples to determine what kind of substances were used, in case an attack, as alleged, did take place.

Following the attack, Syria has sustained coordinated air strikes from the US, the UK and France.

The joint drill was conducted a week after the alleged attack took place.

Russia, one of Syria's main ally, has however slammed the concerned Western nations, accusing the coalition of acting without having enough evidence.

In an interview with BBC's Hardtalk, Russian Foreign Minister Sergei Lavrov also played down US claim that Russia may have tampered with the site to impede investigations.

He said, "I can guarantee that Russia has not tampered with the site."

He also said that all three countries' actions were 'based on media reports and social media'.

Meanwhile, reports from Hezbollah, a Lebanese Islamist political party harbouring pro-Iranian sentiments, said that the Syrian air defence intercepted three missiles targeting Dumair military airport, north-east of Damascus.

US has said that it wasn't behind the attack.

Reuters news agency quoted a Pentagon spokesperson as stating, "There is no US military activity in that area at this time."

Image: UN Photo