The top diplomats in France, Britain and the United States, three of Ukraine’s strongest allies, issued a rare joint statement that rejected Russia’s allegation that Kyiv is preparing to use a so-called dirty bomb on its own territory, calling it a pretext Moscow has concocted for escalating the war.
In the statement, the three governments confirmed that their defense ministers had each spoken with the Russian defense minister, Sergei K. Shoigu, and rejected “Russia’s transparently false allegations” about a dirty bomb.
“The world would see through any attempt to use this allegation as a pretext for escalation,” the statement said.
A so-called dirty bomb uses traditional explosives to spray radioactive material. Russia has not publicly offered evidence to back up the accusations and Ukraine’s foreign minister, Dmytro Kuleba, has called the statements “lies.”
In a separate statement, the British defense secretary, Ben Wallace, said Mr. Shoigu had accused Ukraine’s allies, including Britain, of planning “to escalate the conflict in Ukraine.” Mr. Wallace refuted those claims, the statement said, and “cautioned that such allegations should not be used as pretext for greater escalation.”
Eight months into the war, Russia finds itself on its back foot, struggling to hold onto territory in its ground war and turning to missile strikes that have damaged and destroyed civilian and infrastructure targets far from the frontline. President Vladimir V. Putin of Russia is also facing growing anxiety at home about his military’s handling of the war.
Under that backdrop, Mr. Putin has raised the specter of using nuclear weapons — a terrifying prospect that many Americans have not worried much about since the end of the Cold War — to hold on to his slipping territorial gains in Ukraine. President Biden has warned that the war in Ukraine could devolve into a nuclear “Armageddon,” but the White House has emphasized that the United States has seen no signs that Russia is, in fact, gearing up to use nuclear weapons.
Mr. Shoigu’s conversations in recent days with the top defense officials in France, Britain and the United States were among a flurry of calls he has held in recent days. Defense Secretary Lloyd J. Austin III spoke with Mr. Shoigu on Sunday for the second time in three days, U.S. officials said, in a conversation meant to delineate the red lines that could potentially provoke Russia to launch a nuclear attack on Ukraine.
That was meant to clarify for the Biden administration why Mr. Putin has been increasingly raising the prospect of a nuclear strike in Ukraine, two officials said. With his forces losing ground, Mr. Putin has sought to portray territory in Ukraine he illegally annexed as part of “Mother Russia.” He has said that any American-backed attack inside those areas would be viewed as an attack on the Russian homeland.
Mr. Austin and Mr. Shoigu also spoke on Friday at the request of the Pentagon. Before then, the two last spoke in May.