Russian military vehicles enter the Douma district in the eastern countryside of the capital Damascus of Syria, on April 14, 2018. Around 93 buses with hundreds of Islam Army militants and their families onboard prepared on Friday to evacuate Damascus' eastern Douma district, amid reports of the arrival of the chemical weapons investigators on Saturday to look into allegations of toxic gas use in Douma. (Xinhua/Ammar Safarjalani) (zjl)

Douma - Inspectors from the OPCW chemical watchdog will begin their investigation Sunday at the site of an alleged chemical attack near Damascus, a senior official said.

"The fact-funding team arrived in Damascus on Saturday and is due to go to Douma on Sunday," Deputy Foreign Minister Ayman Soussan told AFP.

A delegation of experts from the Organisation for the Prohibition of Chemical Weapons, based in The Hague, is tasked with investigating an April 7 attack on Douma, just east of Damascus.

Western powers say chemical substances, most likely chlorine and sarin, were used in the attack and killed at least 40 civilians.

The alleged attack, which Damascus and its Russian ally have denied ever happened, prompted an unprecedented wave of missile strikes by the United States, France and Britain on Saturday.

Soussan reiterated a pledge by the Syrian government that the chemical experts would be allow to investigate unimpeded.

"We will ensure they can work professionally, objectively, impartially and free of any pressure," he said.