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"The OPCW executive council has overwhelmingly rejected the Russian and Iranian decision," the British delegation to the watchdog said in a Tweet.

The draft decision put forward by Moscow and Tehran -- and obtained by AFP -- had called for a new investigation by the Organisation for the Prohibition of Chemical Weapons (OPCW) "to establish whether chemical weapons were used in Khan Sheikhun and how they were delivered to the site of the reported incident".

But it ignored that the body, based in The Hague, is already investigating the April 4 attack on the rebel-held town in Idlib province which left 87 dead, including many children.

The draft had also called for investigators to visit the Shayrat airbase -- bombed by the United States after the attack -- to "verify allegations concerning the storage of chemical weapons" there.

But the Russian move had "attempted to undercut" the OPCW's existing fact-finding mission (FFM), the British delegation said in its tweet.

"Needless to say - FFM investigation continues" and "the UK fully supports it," it added.

The move came as OPCW head Ahmet Uzumcu said Wednesday that "incontrovertible" OPCW test results had shown sarin gas or a similar substance were used in the attack.

Sarin confirmed

The move came as OPCW head Ahmet Uzumcu said Wednesday that "incontrovertible" OPCW test results had shown sarin gas or a similar substance were used in the attack.

Samples from three people killed in the attack and seven survivors analysed at four OPCW-designated laboratories "indicate exposure to sarin or a sarin-like substance," said Uzumcu.

Western nations have accused the Syrian regime of carrying out the suspected air strike, but Moscow, the closest ally of Syrian President Bashar al-Assad, has sought to clear the regime of blame.

The Russian-Iranian move for a new inquiry raised hackles at the OPCW executive council meeting this week

'No proof'

"Syria has been accused without any proof of being responsible for using chemical weapons," Russian ambassador Alexander Shulgin told AFP.

"Russia is being accused of covering up Syria's use of chemical weapons. These accusations are baseless and groundless."

He also accused the OPCW of not being "pro-active enough".

"Ten days after the incident they have not yet started with the real investigation. They have not been to the site."

But the Russian-Iranian move for a new inquiry raised hackles at the OPCW executive council meeting this week.

The fact-finding team "deserves our full confidence," the Belgian representative to the OPCW told the meeting on Wednesday.

"We don't see the need to put in place a new structure."

The draft decision had also sought to urge member states to "provide national experts for participation in the investigation."

That would have enabled Moscow to deploy its own experts alongside the OPCW's independent teams in a bid "to discredit the results" so far, one source close to the discussions told AFP.

In an unprecedented step, the OPCW's executive council in November condemned Syria's use of toxic weapons -- the council's first public condemnation of a member of the Chemical Weapons Convention.

It came after a joint UN-OPCW investigation concluded in October that the Syrian air force had dropped chlorine barrel-bombs from helicopters on three opposition-held villages in 2014 and 2015.

Islamic State jihadists were also found to have used mustard gas in August 2015 in Syria.

Russia last week vetoed a UN draft resolution condemning the April 4 attack and demanding the Syrian government cooperate with an investigation, blocking Security Council action against its ally for an eighth time.

After Moscow initially said a Syrian air strike had struck a "terrorist warehouse" containing "toxic substances," Russian President Vladimir Putin last week accused Assad's opponents of planning to stage chemical attacks to lure Washington deeper into the conflict.