LIZ Truss has warned that Vladimir Putin would be making a “grave mistake” if he turned to chemical weapons as part of his continued assault on Ukraine.

Western governments believe the Russian leader could use the weapons, banned under international law, if he grows frustrated at a lack of progress in his invasion of Ukraine.

The UK Government has already accused the Russian government of war crimes, with the bombing of a maternity hospital in Mariupol yesterday being the latest atrocity in the war.

Speaking on a visit to the United Sates, the Foreign Secretary said: “We absolutely believe that war crimes are being committed.”

The UK and US fear Russia could go further and carry out a chemical attack, potentially under the cover of a “false flag” operation.

Asked if the use of such weapons would be a “red line” for the UK, and how it would respond in that situation, Ms Truss told CNN: “We are very concerned about the potential use of chemical weapons.

“Now, of course, we’ve seen Russia use these weapons before in fields of conflict, but that would be a grave mistake on the part of Russia, adding to the grave mistakes that have already been made by Putin.”

White House press secretary Jen Psaki said the world should be “on the lookout” for the Russian use of chemical and biological weaponry.

READ MORE: 'New low': Nicola Sturgeon accuses Putin of committing war crimes in Ukraine

She said that “Russia’s false claims” about alleged US biological weapons labs and chemical weapons development in Ukraine could be an “an obvious ploy” by the Kremlin to try to “justify its further premeditated, unprovoked, and unjustified attack on Ukraine”.

The attack on the Mariupol maternity hospital left three dead, including a child, and has been widely condemned.

UK Defence Minister James Heappey told BBC Radio 4’s Today programme that UK intelligence believes the strike came from artillery rather than the air, but that Britain is “still looking at exactly (what happened)”.

The army veteran said that, even if Russian troops did not deliberately target the medical complex, the attack still amounts to a war crime.

Mr Heappey told BBC Breakfast: “We ask ourselves the question how did this happen? Was it an indiscriminate use of artillery or missiles into a built-up area, or was a hospital explicitly targeted?

“Both are equally despicable, both, as the Ukrainians have pointed out, would amount to a war crime.”

During a call with Mr Zelensky on Wednesday evening, Downing Street said Prime Minister Boris Johnson pointed out that the Mariupol bombing “was yet further evidence” that Mr Putin was “acting with careless disregard for international humanitarian law”.

The World Health Organization said it has confirmed 18 attacks on medical facilities since the Russian invasion began two weeks ago.

But Russia’s foreign minister Sergei Lavrov dismissed concerns about civilian casualties as the “pathetic shrieks” of Moscow’s enemies and claimed the Mariupol hospital had been used as a base by fighters from a far-right group.

Mr Lavrov held a meeting with Ukrainian counterpart Dmytro Kuleba in Turkey but there was no breakthrough to end the fighting.

Mr Kuleba said: “I insisted on the urgent need to allow humanitarian help for Mariupol and a 24-hour ceasefire.

“Unfortunately, FM Lavrov seemed to have come to talk, not to decide. I hope he will convey Ukraine’s requests back in Moscow.”

Efforts continued to allow civilians to escape towns and cities including Mariupol and the Kyiv suburbs.

Conditions in Mariupol are grim, with food and water running short and some of the trapped citizens resorting to melting snow to drink.

Western officials have described the refugee situation in Ukraine as “unprecedented”, with concerns the total number of people fleeing could reach four million within days.

So far around 2.2 million people have left Ukraine.

One official said: “I would like to stress the sheer scale of this, which is something that we haven’t seen, certainly since the end of the Second World War, and that is a real challenge to us all.”