Russia on Tuesday challenged at the UN Security Council the findings of an investigation that blamed the Syrian government for a sarin gas attack as the top United Nations body weighed the future of the panel.

Russian Deputy Ambassador Vladimir Safronkov said the work of the investigative panel was "deeply disappointing" and suggested it was being used by the West to incriminate President Bashar al-Assad's regime.

The Joint Investigative Mechanism (JIM) concluded in a report last month that the Syrian government was responsible for the April 4 sarin attack at Khan Sheikhun that killed scores of people, including children.

Russia, Syria's ally, and the United States have put forward competing draft resolutions on extending the work of the panel after its mandate expires on November 16.  

"We are convinced that the mechanism, endowed with such high responsibility, cannot work in this way," Safronkov told the council.

"Without a comprehensive change, it will become a tool to settle accounts with the Syrian authorities," he said.

Russia maintains the report is not credible because the experts did not go to Khan Sheikhun and worked from samples Moscow maintains may have been tampered with by Western intelligence.

Presenting the findings, Edmond Mulet, the head of the panel, said the experts had established that the sarin was dropped in an air strike and that Syrian aircraft were in the area when the attack took place.

Analysis of the sarin used at Khan Sheikhun matched those of the nerve gas found in Syrian stockpiles, Mulet said, adding that this composition "would be very difficult to replicate."

- Abetting those using chemical weapons -

Safronkov shot back that "the chemicals could have been produced anywhere to compromise intentionally the authorities of Syria."

The United States again appealed for extending the work of the panel that was set up in 2015 to determine who is behind chemical attacks in Syria's six-year war.

"Anyone who prevents us from achieving this goal is aiding and abetting those who have been using chemical weapons in Syria," said US Ambassador Nikki Haley.

"They are helping to ensure, not just that more women and children will die, but that those women and children will die in one of the cruelest, most painful ways possible."

France and Britain came out strongly in support of the findings and said the panel must be allowed to continue its work while China, which works closely with Russia at the council, called for agreement. 

Russia's draft resolution calls for a six-month extension of the panel but said it would "retain its findings" on Khan Sheikhun "until such time when full-scale and high-quality investigation on the site of an incident becomes possible."

The United States circulated a new draft text on Tuesday that called for an 18-month extension, instead of two years as initially proposed.

Russia in late October vetoed a US-drafted resolution that would have extended the mandate of the JIM for a year, casting its ninth veto at the Security Council to block action against its Syrian ally.

Moscow said it wanted to examine the report on Khan Sheikhun before deciding whether it should continue its work.

There has been no request for a vote at the council from either the United States or Russia and diplomats said negotiations were continuing.

Italian Ambassador Sebastiano Cardi told reporters after the meeting that there was strong support for renewing the mandate of the JIM and that agreement was possible.