Russia will station tactical nuclear weapons in neighbouring Belarus, President Vladimir Putin has said, marking the first time since the mid-1990s that Moscow will have based such arms outside the country.
Mr Putin made the announcement at a time of growing tensions with the West over the Ukraine war and as some Russian commentators speculate about possible nuclear strikes.
"Tactical" nuclear weapons refer to those used for specific gains on the battlefield rather than those with the capacity to wipe out cities. It is unclear how many such weapons Russia has, given it is an area still shrouded in traditions of Cold War secrecy.
Experts told Reuters the development was significant, since Russia had until now been proud that unlike the United States, it did not deploy nuclear weapons outside its borders.
Belarus has borders with three NATO members - Poland, Lithuania and Latvia.
"This is part of Putin's game to try to intimidate NATO... because there is no military utility from doing this in Belarus as Russia has so many of these weapons and forces inside Russia," said Hans Kristensen, director of the nuclear information project at the Federation of American Scientists.
Mr Putin told state television that Belarusian President Alexander Lukashenko had long raised the issue of stationing tactical nuclear weapons in his country.
"There is nothing unusual here either: firstly, the United States has been doing this for decades. They have long deployed their tactical nuclear weapons on the territory of their allied countries," he said.
"We agreed that we will do the same - without violating our obligations, I emphasise, without violating our international obligations on the nonproliferation of nuclear weapons."
Mr Putin did not specify when the weapons would be transferred to Belarus.
Russia will have completed the construction of a storage facility for tactical nuclear weapons in Belarus by 1 July.
"We are not handing over (the weapons). And the US does not hand (them) over to its allies. We're basically doing the same thing they've been doing for a decade," Mr Putin said.
"They have allies in certain countries and they train... their crews. We are going to do the same thing."
The US State Department and the Pentagon did not immediately return messages seeking comment.
Russia has stationed 10 aircraft in Belarus capable of carrying tactical nuclear weapons, Mr Putin said, adding that Moscow had already transferred to Belarus a number of Iskander tactical missile systems that can launch nuclear weapons.
"It's a very significant move," said Nikolai Sokol, a senior fellow at the Vienna Center for Disarmament and Non-Proliferation.
"Russia had always been very proud that it had no nuclear weapons outside its territory. So, now, yes, they are changing that and it's a big change."
When the Soviet Union collapsed in 1991, nuclear weapons were deployed in the four newly-independent states of Russia, Ukraine, Belarus and Kazakhstan.
In May 1992, the four states agreed all the weapons should be based in Russia and the transfer of warheads from Ukraine, Belarus and Kazakhstan was completed in 1996.
IAEA head to visit Zaporizhzhia nuclear power plant
The move comes as the UN nuclear watchdog chief, Rafael Grossi, announced he will visit the Russian-held Zaporizhzhia nuclear powerplant in Ukraine next week to assess the serious situation there.
Mr Grossi is pressing for a security zone to be erected around what is Europe's largest nuclear power plant, with six reactors, which has come under repeated shelling over the past months.
It will be his second visit.
Last September he went there and established a permanent presence of IAEA experts.
Russian troops occupied the facility early in their invasion of Ukraine and it remains near the front line.
Both sides blameeach other for the shelling.
"The situation at the Zaporizhzhia nuclear power plant is still precarious," Mr Grossi said in the statement, saying he wanted "to assess first-hand the serious nuclear safety and security situation at the facility".
Earlier this month he appealed for the protection zone around the plant to be set up, saying he was "astonished by the complacency" around the issue.
The plant accounted for around 20% of Ukraine's national power generation before the invasion, but has not produced any electricity since September when the last of its six reactors were taken offline.