Syrian Foreign Minister Walid Muallem said the UN mission investigating alleged chemical weapons attacks in Damascus has been delayed until Wednesday after rebels failed to guarantee the experts' safety.

"Today, we were surprised by the fact that they were not able to get there because the rebels did not agree to guarantee the mission's security. So the mission has been delayed until tomorrow," Muallem told a televised news conference.

The delay was confirmed by UN spokesman Farhan Haq, referring to a safety concerns after a sniper attack on the inspector's convoy on Monday.

"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 opposition denied regime allegations that rebels were failing to guarantee the inspectors' safety.

"The regime is trying to convince the mission that the lives of its members will be in danger if they travel to Eastern Ghouta," where some of the attacks allegedly occurred, a statement from the rebels said.

"The military council has contacted some of them to convince them of the regime's false allegations and to assure them that the council is totally committed to provide them protection." (AFP)