The Syrian army said that an air strike late on Wednesday by the U.S.-led coalition hit poison gas supplies belonging to the Islamic State group, releasing a toxic substance that killed "hundreds including many civilians." The U.S.-led coalition denied the report.

The incident in the eastern Deir al-Zor province proved that ISIS and Al-Qaida-linked militants "possess chemical weapons," a statement by the army flashed on Thursday by Syrian state TV said, a week after the country drew widespread condemnation for a chemical attack that killed at least 80 people.

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U.S. Air Force Colonel John Dorrian, a spokesman for the coalition, said it had carried out no air strikes in that area at that time.

"The Syrian claim is incorrect and likely intentional misinformation," he said in an email to Reuters.

The Russian defence ministry has no information about an attack carried out by international coaltion forces in Syria's province of Deir al-Zor, Igor Konashenkov, the defence ministry's spokesman said on Thursday, RIA news agency reported.

Konashenkov said Russian forces sent drones to check the area.

The Syrian claim came as the international community, with the exception of Russia, has condemned the regime of Bashar Assad for using chemical weapons against civilian targets in its fight against rebel forces.

Assad's forces allegedly deployed sarin gas in an attack that killed scores last week in the northwestern province of Idlib, an area held by rebels.

The U.S. responded to the incident by launching 59 cruise missiles at the airforce base where the planes responsible for the attack had taken off.

On Wednesday, Britain's UN envoy said samples had shown that sarin gas had been used in the attack. Both the U.S. and Britain consider it highly likely that Assad was responsible for the attack, as it's considered unlikely that rebel forces control any chemical assets.

After the attack, Israel's Defense Ministery Avigdor Lieberman said he was "100 percent certain" that Assad was behind the attack and the U.S. has made a preliminary conclusion that Russia knew in advance that chemical weapons would be used in the strike.

A senior U.S. official said, however, that no proof has yet emerged that suggests Russia had direct involvement in the attack.

Meanwhile, U.S.-backed forces fighting Islamic State in Syria launched a new phase of their offensive on Thursday, a statement said, but they have not yet begun to attack the militant group's stronghold of Raqqa city in an apparent delay in the operation.

The multi-phased campaign by the Syrian Democratic Forces (SDF), an alliance made up of Syrian Kurdish and Arab fighting groups, was launched in November and aims ultimately to drive the jihadists from Raqqa, their de facto Syrian capital.

The multi-phased campaign by the Syrian Democratic Forces (SDF), an alliance made up of Syrian Kurdish and Arab fighting groups, was launched in November and aims ultimately to drive the jihadists from Raqqa, their de facto Syrian capital.