KYIV, Oct 23 - Russia’s defense chief alleged Sunday that Ukraine was preparing a “provocation” involving a radioactive device, a stark claim that was strongly rejected by U.S., British and Ukrainian officials amid soaring tensions as Moscow struggles to stem Ukrainian advances in the south.

Russian Defense Minister Sergei Shoigu made the allegations in phone calls with his counterparts from the United States, Britain, France and Turkey, The Associated Press reported.

Russia’s defense ministry said Shoigu voiced concern about “possible Ukrainian provocations involving a ‘dirty bomb,’” a device that uses explosives to scatter radioactive waste. It doesn’t have the devastating effect of a nuclear explosion, but could expose broad areas to radioactive contamination.

Russian authorities repeatedly have made allegations that Ukraine could detonate a dirty bomb in a false flag attack and blame it on Moscow. Ukrainian authorities, in turn, have accused the Kremlin of hatching such a plan.

British Defense Secretary Ben Wallace strongly rejected Shoigu’s claim and warned Moscow against using it as a pretext for escalation.

The British Ministry of Defense noted that Shoigu, in a call with Wallace, “alleged that Ukraine was planning actions facilitated by Western countries, including the UK, to escalate the conflict in Ukraine.”

“The Defense Secretary refuted these claims and cautioned that such allegations should not be used as a pretext for greater escalation,” the ministry said.

The U.S. also rejected Shoigu’s “transparently false allegations,” White House National Security Council spokesperson Adrienne Watson said in a statement. “The world would see through any attempt to use this allegation as a pretext for escalation.”

In a televised address Sunday evening, President Volodymyr Zelensky suggested that Moscow itself was setting the stage for deploying a radioactive device on Ukrainian soil.

“If Russia calls and says that Ukraine is allegedly preparing something, it means only one thing: that Russia has already prepared all of it,” Zelensky said.

The rising tensions come as Russian authorities reported building defensive positions in occupied areas of Ukraine and border regions of Russia, reflecting fears that Ukrainian forces may attack along new sections of the 1,000-kilometer (620-mile) front line of the war, which enters its ninth month on Monday.

In recent weeks, Ukraine has focused its counteroffensive mostly on the Kherson region. Their relentless artillery strikes cut the main crossings across the Dnieper River, which bisects the southern region, leaving Russian troops on the west bank short of supplies and vulnerable to encirclement.

About 20,000 Kherson residents have moved to places on the east bank of the Dnieper River, the Kremlin-backed regional administration reported. The Ukrainian military said Sunday that Russia’s military also withdrew its officers from areas on the west bank, leaving newly mobilized, inexperienced forces. (ap/ez)