The United Nations has postponed a planned new visit by chemical weapons experts to a site near Damascus on Tuesday over "safety" fears, a spokesman said.

A sniper attack on the experts Monday forced the review of the plan to return to the Damascus suburb of Ghouta, said UN spokesman Farhan Haq.

"Following yesterday's attack on the UN convoy, a comprehensive assessment determined that the visit should be postponed by one day in order to improve preparedness and safety for the team," said Haq.

The UN team has still not obtained "confirmation of access" for a new visit but Haq said this was expected later Tuesday. He gave no other details of what was holding up the confirmation.

UN leader Ban Ki-moon "again urges all sides in the conflict to give safe passage and access to the team," said the spokesman.

"It is in the interests of all sides to bring factual evidence and clarity to a situation which has brought great suffering to the people of Syria."

The suspected chemical weapons attack on Ghouta on August 21 left hundreds dead, according to opposition activists. Some Western nations have already blamed President Bashar al-Assad for the use of the banned arms and are now reportedly considering a military strike.