(LONDON) — Tens of thousands of civilians could be dead in Mariupol, the city’s mayor said, as analysts warn that Russia is regrouping for a renewed assault on eastern Ukraine.

While there is no confirmed number of casualties, Vadym Boychenko, the mayor of Mariupol, suggested in an interview that the number of dead could be well over 10,000 in the coastal town, the site of some of the worst bombardment since the Russian invasion of Ukraine began.

Russian airstrikes have battered the southeastern city over the past few weeks targeting all kinds of buildings, including a theatre housing those seeking refuge, a maternity hospital and an art school. Officials say that over 80 percent of the city is destroyed.

With the city almost completely cut off from the outside world, it has been difficult for the authorities to verify atrocities alleged to have been committed on the ground, including claims made Monday by the Azov battalion, a far-right group now part of the Ukrainian National Guard, that Russia may have used chemical weapons against the Ukrainian forces in the city.

The claims could not be independently verified by ABC News.

Ukrainian President Volodymyr Zelenskyy’s office and the country’s ministry of defense said they are investigating the claims.

“We are currently confirming this information, trying to understand what it was. According to preliminary information, we can say that it is possible– that it was, rather, a phosphorous munition. Official information will be finalized later,” Deputy Minister of Defence Hanna Maliar said during a national news broadcast.

A spokesman for Russian-backed separatists, Eduard Basurin, was quoted by the Interfax news agency as saying Tuesday that they “haven’t used any chemical weapons in Mariupol.”

But on the eve of the alleged attack, Basurin made the case on Russian TV that his forces should use chemical weapons against Ukrainian troops to “smoke them out.”

Late on Monday, Pentagon spokesman John Kirby issued a statement saying that the use of a substance cannot be confirmed, but expressed concerns about Russia’s potential use of “riot control agents including tear gas mixed with chemical agents.”

“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. These reports, if true, are deeply concerning and reflective of concerns that we have had about Russia’s potential to use a variety of riot control agents, including tear gas mixed with chemical agents, in Ukraine,” the statement reads.

Britain’s armed forces minister said on Tuesday that should Russia resort to the use of chemical weapons in the eastern Ukrainian city of Mariupol, “all possible options are on the table in terms of how the West might respond” in response to the speculation, saying the reports had not been verified.

Whether or not phosphorous is considered a chemical weapon is a topic of debate among militaries and governments.

Although phosphorus is not classified as a chemical weapon under the Chemical Weapons Convention, the Geneva Convention forbids its use as an incendiary weapon in civilian areas.

Amidst the reports of Russia regrouping behind its own borders, the British Ministry of Defense warned today that the fighting is set to intensify over the next two-three weeks with a renewed assault.

“Fighting in eastern Ukraine will intensify over the next two to three weeks as Russia continues to refocus its efforts there,” the U.K.’s ministry of defense stated 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 towards Kramatorsk. Russian forces continue to withdraw from Belarus in order to redeploy in support of operations in eastern Ukraine,” the post added.

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