United Nations' vehicles leave the hotel where the international experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) are staying in Damascus on April 18, 2018

A UN security team was fired at on Tuesday while on a reconnaissance mission in the Syrian town of Douma to prepare the deployment of experts investigating an alleged chemical attack, officials said.

"Shots were fired yesterday at a UN security team doing a reconnaissance in Douma," a UN official told AFP. "They were not injured and returned to Damascus."

The experts from the OPCW chemical watchdog were awaiting the green light from the security team before beginning their on-site investigation in Douma of the alleged attack.

The suspected April 7 gas attack on Douma, near Damascus, reportedly left more than 40 people dead and was blamed by Western powers on Syrian President Bashar al-Assad's forces.

British Ambassador Peter Wilson told reporters in The Hague that the security team on Tuesday traveled to two sites in Douma, escorted by Russian security police.

The officials were greeted by a "large crowd" of protesters at one site, forcing them to withdraw while at the second site "they were subject to small-arms fire and an explosion," Wilson said, citing information from the OPCW director general.

Ahmet Uzumcu, the head of the Organisation for the Prohibition of Chemical Weapons, told ambassadors in The Hague that it was unclear when the fact-finding mission will be able to deploy to Douma, Wilson added.

The UN official, who declined to be named, said the security team would remain in Damascus with the OPCW experts for the time being.

Asked whether the team was specifically targeted, Wilson said, "I can't answer that question. It's very much a question for the OPCW."

The inspectors arrived in Damascus on Saturday, when Britain, France and the United States launched military strikes against what they said were targets linked to Syria's chemical weapons program.

Syria's UN Ambassador Bashar Jaafari told the Security Council on Tuesday that the OPCW experts would begin their investigation once they received the all-clear from the UN security team.

"If this United Nations security team decides that the situation is sound in Douma, then the fact-finding mission will begin its work in Douma tomorrow," Jaafari said.

The ambassador stressed that the "Syrian government did all that it can do to facilitate the work of this mission" but that it was up to the United Nations and the OPCW to decide whether to deploy, based on security considerations.