London, Apr 12 (IANS): A pro-Putin separatist leader has called for Russia to use chemical weapons to 'smoke out' 4,000 Ukrainian defenders at a key strategic steel plant in Mariupol, the media reported.

Eduard Basurin, deputy head of the so-called Donetsk People's Republic, claimed up to 4,000 Ukrainian fighters are taking shelter in the Azovstal complex, one of Europe's largest iron and steel works, which has so far defied Russian attempts to seize it, the Daily Mail reported.

He made the dire warning just hours after Ukraine accused Russia of unleashing a toxic agent from a drone over Mariupol, causing breathing issues, Daily Mail reported.

This morning, the Azov Battalion showed footage purporting to show victims of the apparent attack, who said they have since suffered respiratory problems, high temperatures and tinnitus.

Making further threats on Russian state TV, Basurin said today: "What is Azovstal? It is a plant built back in Soviet times. There is a lot or concrete, iron, there are many underground floors. So it makes no sense to take this facility by storm.

"Therefore, at the moment it is necessary to deal with the blocking of this plant, find all the exits and entrances. In principle, this can be done.

"And after that, to turn, I think, to the chemical troops, who will find a way to smoke moles out of their holes."

Yesterday's alleged chemical attack has not been confirmed but would be the first time Russia is believed to have used toxic weapons in Putin's barbaric invasion, which the Pentagon said is 'deeply concerning'.

US Defence Department spokesman John Kirby said: 'We are aware of social media reports which claim Russian forces deployed a potential chemical munition in Mariupol, Ukraine. We cannot confirm at this time and will continue to monitor the situation closely.