The United States and Britain accused Russia on Thursday of "denying the truth" as Moscow sought to rip up a report blaming Syria for a deadly chemical attack in April.

In a strongly-worded statement at a meeting of the watchdog Organisation for the Prohibition of Chemical Weapons (OPCW), US representative Ken Ward said Russia was "continuing to deny the truth" regarding the attack in which sarin gas was used.

Last month a report by a joint UN-OPCW panel said the Syrian air force on April 4 had dropped a bomb on the opposition-held town of Khan Sheikhun, releasing the deadly nerve agent that killed more than 80 people, including children.

But Russia has dismissed the Joint Investigative Mechanism (JIM) report, saying the experts did not travel to Khan Sheikhun and worked with samples Moscow maintains may have been tampered with by Western intelligence.

Russia also lambasted the investigators for not travelling to the Shayrat air base, from where the aerial attack on the northwestern Syrian town was allegedly launched.

But Ward accused Russia of "collaborating with the (Bashar al-)Assad regime in a deplorable attempt to discredit the (OPCW) Fact Finding Mission (FFM) and the Joint Investigative Mechanism."

"Regrettably, Russia has abetted Syria's use of chemical weapons and disregarded the international responsibilities Syria undertook," Ward said in a statement posted on the OPCW's website.

The dispute came as Russia and Iran, the Syrian regime's allies, proposed a resolution at the OPCW meeting on Wednesday which diplomats say effectively asks investigators to scrap the first probe into the Khan Sheikhun incident and launch a new investigation.

The draft resolution "decides to renew the work by the FFM on the incident with the use of chemical weapons in Khan Sheikun in order to ensure a full-scale, professional and high-quality investigation to identify sarin," according to a copy seen by AFP.

It also asks for the fact-finding team to hold back the results of its new probe "until (a) full-scale investigation on the site of the incident in Khan Sheikhun becomes possible".

Britain's permanent representative to the OPCW Peter Wilson said London and other Western countries stood behind the original report and accused Moscow of wanting "to confuse the delegates of the OPCW".

The investigators "have not taken any evidence they can't corroborate from at least three sources," Wilson told AFP.

Earlier in his statement to the OPCW's Executive Council meeting -- of which Russia and the United States are also members -- Wilson accused Russia and Iran of "seeking to politicise" the OPCW.

"It is a thinly veiled political attack on the professional integrity" of the OPCW's director general Ahmet Uzumcu aimed at undermining the organisation's technical capability and competence, Wilson said.

Canada also joined the fray, saying that Syria, which joined the Chemical Weapons Convention in 2013 under joint US-Russia pressure, "forfeited the rights and privileges of membership, and should no longer be permitted to sit among us."