Russia’s claims that Ukraine is preparing a “dirty bomb” – an explosive laced with radioactive material – are unsubstantiated, Kyiv said on Sunday, the BBC reported.

Russian Defense Minister Sergei Shoigu made the allegation about a possible “dirty bomb” attack in a telephonic conversation with his British counterpart Ben Wallace.

Ukraine President Volodymyr Zelenskyy said that Moscow could be planning an attack of this kind.

He said Russia was threatening the world “with a radiation disaster” at the Zaporizhzhia nuclear plant in Ukraine. He also claimed that Moscow wanted to detonate a major dam.

“I believe that now the world should react as harshly as possible,” Zelenskyy said, The Guardian reported.

Moscow has denied all the allegations.

Meanwhile, the United States has rejected Russia’s claims, saying the country has no evidence of a dirty bomb.

Secretary of State Antony Blinken told his Ukrainian counterpart, Dmytro Kuleba, on Sunday “the world would see through any attempt by Russia to use this allegation as a pretext for escalation”, and vowed to continue supporting Kyiv for as long as necessary.

The United Kingdom’s Ministry of Defence in a statement rebuked Russia for saying that Ukraine was planning its actions with help from Western countries. “The defence secretary refuted these claims and cautioned that such allegations should not be used as a pretext for greater escalation,” it said.

Russia had launched an invasion of Ukraine on February 24, describing its actions as a “special operation” to demilitarise and “de-Nazify” Ukraine. However, Ukraine and several Western countries said this was a baseless pretext for a war of choice by Russian President Vladimir Putin.