Russia says the United States and its Western allies rushed to judgment and blamed the Syrian government for using sarin nerve gas in an attack on an opposition-held town in Syria without ever visiting the site and ignoring two witnesses presented by Damascus.

It also criticised the report by the fact-finding mission from the chemical weapons watchdog that investigated the incident in Khan Sheikhoun on April 4 which killed more than 90 people, calling it "very biased." 

A letter from Russia's UN Mission to Secretary-General Antonio Guterres and the UN Security Council circulated Wednesday contains Russia's assessment of the status of the investigation into the incident in Idlib province.

The attack sparked outrage around the world as photos and video of the aftermath, including quivering children dying on camera, were widely broadcast.

The United States blamed the Syrian military for the attack and launched a punitive strike days later on the Shayrat air base where it said the attack was launched. Syrian President Bashar Assad, a Russian ally, has denied using chemical weapons.

The Russian assessment says one thing is clear sarin was used and this was confirmed by analysis of samples from the site received by Syrian authorities.

"However, one important question remains unanswered by whom, in what circumstances and how it was used," the Russian report said.

A fact-finding team from the Organization for the Prohibition of Chemical Weapons investigated the Khan Sheikhoun attack and reported on June 30 that sarin was used.

It is now up to a joint UN-OPCW body known as the Joint Investigative Mechanism or JIM to determine responsibility.

"It will be impossible to establish the truth without a visit to Khan Sheikhoun by the experts of the fact-finding mission and the Joint Investigative Mechanism, although the perpetrators and organizers of this horrific act of provocation have already done a lot of cleaning up and manipulation," the Russian report said.

It said an inspection of the Shayrat airfield "is still a priority, as this facility (was) allegedly the storage site for the sarin used in Idlib." 

The report said the Syrian government "offered guarantees" of full access to Shayrat and invited the OPCW fact-finding mission and the JIM to visit the site immediately after "the tragic events" in Khan Sheikhoun. But it said the OPCW didn't visit because of "the security risks involved" and the JIM needed to wait for the fact-finding mission's report.

Russia said investigations by Turkey, Britain and France, which "resulted in quite predictable and peremptory findings of guilt on the part of Damascus," raised many questions including on whether the collection of evidence met international standards.

And Russia asked why the three countries managed to obtain evidence from the site but didn't take the opportunity to visit Khan Sheikhoun.

As for the fact-finding mission's report, Russia said it "intentionally" emphasized that the statements from the two witnesses presented by Syrian authorities supported a divergent view that it was "an act of provocation by the militants and their foreign sponsors." 

"Should we not seek the truth of the events in these same statements, which differ from the general picture that has clearly been orchestrated by the opposition and by NGOs sympathetic to the opposition?" Russia asked.

"Perhaps we should first of all seek to investigate the 'real estate agents' from an armed opposition group, who rented out a building for the storage of toxic chemicals, and the ambulances from a neighboring country that removed the injured persons, perhaps as prearranged, from the scene of the chemical attack?" it asked.