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

The forward team had been assessing the security situation when they came under fire, though none were injured.

Peter Wilson, Britain's ambassador to the UN, told reporters in The Hague that the security team on Tuesday travelled 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," Mr Wilson said.

The Organisation for the Prevention of Chemical Weapons (OPCW) inspectors arrived in Syria on Saturday, but have so far been unable to reach the site of the April 7 attack to carry out tests.