DAMASCUS (AFP) - Inspectors from the OPCW chemical watchdog will begin their investigation Sunday (April 15) 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 Agence France-Presse.

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.