The Organisation for the Prohibition of Chemical Weapons has been "working in close collaboration" with UN security experts "to assess the situation and ensure the safety of the team," it said.

It vowed in its statement that the fact-finding mission due to go to Douma later Saturday "will continue its deployment to the Syrian Arab Republic to establish facts around the allegations of chemical weapons use in Douma".

The United States, Britain and France carried out a wave of strikes against the Syrian regime on Saturday a week after the suspected deadly gas attack on the rebel-held Damascus suburb of Douma, in which 40 people were said to have been killed.

In the biggest foreign military action so far against Syria's regime, Western officials said a barrage of cruise and air-to-land missiles hit targets near Damascus and in Homs province including a scientific research centre, storage facilities and a command post.

Syrian state media reported that internal security forces had entered Douma on Saturday and that the town would be secured within hours.

Both Syria and its ally Russia have said they would guarantee the safety and security of the OPCW mission -- the first outside Damascus since 2014.