The city’s Azov regiment reported soldiers were left dizzy and unable to breathe after a “poisonous substance of unknown origin” was dropped on them from a Russian drone, according to reports.

It came hours after Mariupol’s mayor said more than 10,000 civilians have died in the Russian siege of his city and the death toll could surpass 20,000 adding that corpses now “carpeted through the streets” of the city.

The south-eastern port city of Mariupol has seen some of the heaviest attacks and civilian suffering in the six-week war, but the land, sea and air assaults by Russian forces fighting to capture it have increasingly limited information on circumstances inside the city.

Children walk in Bucha on the outskirts of Kyiv, Ukraine, Friday, April 8, 2022. (AP Photo/Rodrigo Abd, File)

Britain is increasingly worried that Russia could use white phosphorus munitions in the bombardment of the city.

The UK Ministry of Defence said Russian forces are continuing to pull out of Belarus to support operations in eastern Ukraine, focused on the Donbas region, where Russian-allied separatists have claimed independence.

“Fighting in eastern Ukraine will intensify over the next two to three weeks as Russia continues to refocus its efforts there,” the ministry said in a tweet.

“Russian attacks remain focused on Ukrainian positions near Donetsk and Luhansk with further fighting around Kherson and Mykolaiv and a renewed push toward Kramatorsk.”

Ukraine conflict: Warning Russia could use phosphorus munitions in Mariupol: Wha...

Defence minister James Heappey if chemical weapons“are used at all then President Putin should know that all possible options are on the table in terms of how the West might respond.

He said that any response “would certainly be effective, but it would also be well considered”.

The minister told Sky News: “We know that there are reports of the use of chemical weapons. We have not been able to verify those ourselves, and indeed the Ukrainian system, as you’ve seen from President Zelensky, are only referring to the fact that there are reports, they themselves haven’t yet been able to confirm to us that they have been used.”

Mr Heappey added: “These are appalling weapons to even think about using and the fact that they are part of the discussion is deeply sobering. It’s not just the president of the United States, the president of France and our own Prime Minister have also been clear that there are weapons that simply should not be used and if they are used people will be held to account.

“It’s important to recognise that there are all sorts of ways in which these things could be used, from the use of tear gas which is effectively a riot-control measure, all the way through to utterly devastating lethal chemical weapons systems, so I don’t think it’s helpful to be too binary about the situation because these are highly nuanced.

“There are some things that are beyond the pale and the use of chemical weapons will get a response, and all options are on the table for what that response could be.”

The UN reports 4.8 million of Ukraine's 7.5 million children have been displaced since the start of the war.

Ukraine conflict: Use of chemical weapons “will get a response" from the West says UK defence minister