Russia has about 2,000 working tactical nuclear weapons while the United States has around 200 such weapons, half of which are at bases in Italy, Germany, Turkey, Belgium and the Netherlands.

Amid the Ukraine crisis, Politico reported on Oct. 26 that the United States told a closed NATO meeting this month that it would accelerate the deployment of a modernised version of the B61, the B61-12, with the new weapons arriving at European bases in December, several months earlier than planned.

"We cannot ignore the plans to modernize nuclear weapons, those free-fall bombs that are in Europe," Russian Deputy Foreign Minister Alexander Grushko told state RIA news agency.

The 12-ft B61-12 gravity bomb carries a lower yield nuclear warhead than many earlier versions but is more accurate and can penetrate below ground, according to research by the Federation of American Scientists published in 2014.

"The United States is modernizing them, increasing their accuracy and reducing the power of the nuclear charge, that is, they turn these weapons into 'battlefield weapons', thereby reducing the nuclear threshold," Grushko said.

The Pentagon said it was not going to discuss the details of the U.S. nuclear arsenal and that the premise of the Politico article was wrong as the United States had long planned the modernisation of its B61 nuclear weapons.

"Modernization of U.S. B61 nuclear weapons has been underway for years, and plans to safely and responsibly swap out older weapons for the upgraded B61-12 versions are part of a long-planned and scheduled modernization effort," Pentagon spokesman Oscar Seara said.

"It is in no way linked to current events in Ukraine and was not sped up in any way," Seara said in an emailed statement.

Russia's invasion of Ukraine has triggered the gravest confrontation between Moscow and the West since the 1962 Cuban Missile Crisis when the two Cold War superpowers came closest to nuclear war.

President Vladimir Putin has repeatedly said Russia will defend its territory with all available means, including nuclear weapons, if attacked.

The comments raised particular concern in the West after Moscow declared last month it had annexed four Ukrainian regions that its forces control parts of. Putin says the West has engaged in nuclear blackmail against Russia.

'STRATEGIC SIGNIFICANCE'

U.S. President Joe Biden said on Oct. 6 that Putin had brought the world closer to "Armageddon" than at any time since the Cuban Missile Crisis, though Biden later said he did not think that Putin would use a tactical nuclear weapon.

Putin has not mentioned using a tactical nuclear weapon but has said he suspects Ukraine could detonate a "dirty bomb", a claim Ukraine and the West say is false.

The U.S. B61 nuclear bomb was first tested in Nevada shortly after the Cuban Missile Crisis. Under Barack Obama, U.S. president from 2009 to 2017, the development of a new version of the bomb, the B61-12, was approved.

Russia's Grushko said that Moscow would also have to take account of the Lockheed Martin F-35 which would drop such a bomb. NATO, he said, had already strengthened the nuclear parts of its military planning.

NATO "has already made decisions to strengthen the nuclear component in the alliance's military plans," Grushko said.

Russia's ambassador to Washington, Anatoly Antonov, said on Saturday on Telegram that the new B61 bombs had a "strategic significance" as Russia's tactical nuclear weapons were in storage, yet these U.S. bombs would be just a short flight from Russia's borders.

The United States, according to the U.S. 2022 Nuclear Posture Review published on Thursday, will bolster nuclear deterrence with the F-35, the B61-12 bombs and a nuclear-armed air-launched cruise missile.