Russia on Thursday accused Western nations of using "naive" allegations of chemical weapons attacks by the Syrian government to block a UN investigation into the use of the arms.

Russia's UN ambassador Vitaly Churkin angrily attacked Western members of the UN Security Council after it emerged that Moscow had blocked a draft resolution backing an investigation.

Russia -- the main ally of Syrian President Bashar al-Assad -- has backed Syria's demand that a UN panel concentrate on an alleged sarin gas attack by Syrian opposition rebels in March.

Britain, France and the United States have backed UN leader Ban Ki-moon's call for "unfettered" access to Damascus to investigate all allegations.

The Western countries say they have handed over evidence to the UN that Assad's forces have used chemical arms in the conflict, which erupted in March 2011.

"Unfortunately I think what our Western colleagues have been doing is trying to produce the maximum number of allegations with minimum credibility in an effort, one might think, to create maximum problems for arranging such an investigation," Churkin told reporters.

Churkin this week handed to Ban the results of a Russian inquiry into an alleged chemical weapons attack near Aleppo on March 19.

He said it showed that a Syrian rebel group carried out the attack in which 26 people, including 16 government soldiers, were killed.

Western diplomats have cast doubt on the government and Russian version of events.

A senior Western diplomat, speaking on condition of anonymity, said Western nations believe the shell was fired by government forces and "fell short and landed in an area where there were Syrian troops, not that the opposition had done it."

Churkin condemned that account as "naive" and a "bizarre scenario".

He also accused Western countries of deliberately leaking talks about a resolution proposed by Britain.

The draft resolution was based on a statement agreed by the Group of Eight summit, including Russia, in Belfast last month, which urged support for an "objective investigation into reports of the use of chemical weapons."

Churkin called the proposed resolution "completely unreasonable and contrary to the G8 declaration" and said he had the support of China in opposing the draft.

The head of the UN chemical weapons experts, Ake Sellstrom, and UN disarmament envoy Angela Kane are expected to go to Damascus soon for talks on the mission.

There are still widespread doubts however about the access they will secure for Sellstrom's team.

Syria has blocked the UN experts, saying they should only investigate the Aleppo incident in March. The UN has indicated its call for unfettered access could be eased.

"The secretary general wishes to emphasize that in light of the seriousness of allegations of the use of chemical weapons, the mission will continue its activities which are aimed at establishing the facts in a credible manner," said UN spokesman Martin Nesirky.

"In accordance with guidelines and procedures approved by the General Assembly there can be no substitute for an on-site investigation of all relevant locations in the Syrian Arab Republic," Nesirky told a press briefing.