Defense Secretary Lloyd J. Austin III spoke with his Russian counterpart 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.
The conversation with the Russian defense minister, Sergei K. Shoigu, at 7:30 a.m. Eastern was meant to clarify for the Biden administration why President Vladimir V. Putin has been increasingly raising the specter of a nuclear strike in Ukraine, two officials said. With his forces on the back foot there, Mr. Putin has sought to portray territory in Ukraine he illegally annexed as part of “Mother Russia,” saying that any American-backed attack inside those areas would be viewed as an attack on the Russian homeland.
Russia’s defense ministry confirmed the two men spoke, saying only that they had discussed the situation in Ukraine.
Mr. Austin and Mr. Shoigu spoke on Friday at the initiation of the Pentagon. Before that, the two last spoke in May.
One U.S. official said on Sunday that Moscow’s turn to Iranian drones, coupled with Mr. Putin’s increasingly escalatory references to nuclear weapons, meant that a desperate Russia was looking for other tools in its arsenal to use. Mr. Putin’s forces have faced struggles on the battlefield in Ukraine and there is growing anxiety at home about his military’s handling of the war.
The conversation with Mr. Austin was among a flurry of calls Mr. Shoigu held with other top defense officials on Sunday. In talks with his British, French and Turkish counterparts, Mr. Shoigu raised concerns about the possibility that Ukraine would use a “dirty bomb,” according to Russia’s Defense Ministry. Russia publicly offered no evidence to back up its claims, and the allegations drew a swift and fierce response from Ukrainian and Western officials.
Ukraine’s foreign minister, Dmytro Kuleba, called them “lies” that were “as absurd as they are dangerous.”
“We neither have any ‘dirty bombs’, nor plan to acquire any,” he wrote on Twitter.
A so-called dirty bomb would use traditional explosives to spray radioactive material, and Russia previously has warned of the possibility that one could be featured in the Ukraine war.
The White House called Mr. Shoigu’s claims “transparently false,” while the Pentagon said Mr. Austin “rejected any pretext for Russian escalation.”
Britain’s defense ministry similarly refuted the claims.
President Biden has taken pains so far to avoid escalating the nuclear brinkmanship, and has indicated that he still has no plans to directly involve American troops — or the U.S. nuclear arsenal — in a war with Russia. At the same time, American officials say the United States would be hard-pressed not to respond in some way if Mr. Putin uses a nuclear weapon in the Ukraine war.
Ukraine’s president, Volodymyr Zelensky, urged more support from the West given Mr. Putin’s nuclear threats.
“When a terrorist state raises the stakes, it must feel that it won’t work,” he said in his nightly address.