Russia may be paving the way to conduct a nuclear test, a move that would sharply raise tensions with the West and likely prompt other world powers to resume testing for the first time this century.
President Vladimir Putin last week said Russia's parliament should consider withdrawing Moscow's ratification of the 1996 Comprehensive Test Ban Treaty (CTBT) which prohibits tests involving nuclear explosions. Parliamentary leaders were due to discuss the issue on Monday.
Some Western security analysts now see a growing likelihood of a Russian test, even though Putin said the aim was only to mirror the position of the United States, which has signed but not ratified the treaty.
"A Russian nuclear test is clearly very much on the cards now. I don't think it's a certainty, but it shouldn't surprise anybody if that happens," said James Acton, co-director of the nuclear policy program at the Carnegie Endowment for International Peace.
Matthew Harries, director of proliferation and nuclear policy at the RUSI think-tank in London, said cancelling Russia's ratification would create a "legal and presentational framework for Russia to test if it wants to".
If Moscow did conduct a test, he said, "it would be a strong form of signalling, to put the nuclear threat in people's minds, to try to signal resolve and to evoke fear".
Former Soviet and Russian diplomat Nikolai Sokov went further, saying a Russian nuclear test would mark a very serious escalation towards actually using an atomic weapon.
For that reason, Sokov said, he did not expect Russia to test at this point. Rather, he said, rescinding ratification would be a political step and part of a wider review of Moscow's security obligations to remove perceived imbalances and "level the field" with the United States.
"At the moment I see a nuclear test as unlikely but the situation is very tense and an escalation is not impossible," said Sokov, now senior fellow at the Vienna Center for Disarmament and Non-Proliferation.
He said he could envisage a Russian nuclear test as part of a scenario where the Ukraine war was going very badly for Putin.
"The trick is to get very close to nuclear use but avoid it - to force the adversary to take a step back so the US and Nato think: 'Is it really worth it?' To change the calculation of costs and benefits," Sokov said in a telephone interview.
"At some point you may need to show that you're very, very serious about possibly using nuclear weapons. That's where a nuclear underground test can play a big role."
Sokov said he did not believe that Putin was interested in actually using nuclear weapons, but the risk in such a scenario was that "you simply can lose control of events" and the logic of escalation could lead him to use them even if that was not his intention at the start.
Russia has not conducted a nuclear test since 1990, the year before the collapse of the Soviet Union. It ratified the CTBT in 2000. Although the treaty has not formally come into force because others including the United States, China, India, Pakistan and Israel have not ratified, it has effectively made nuclear testing taboo. Only North Korea has conducted a test involving a nuclear explosion this century.
Acton said it was plausible, as Sokov said, that Putin was preparing the option of a nuclear test as a warning signal if things went badly for him in Ukraine. But he said it was also possible that Putin had already decided he was going to test, irrespective of what was happening in the war.
In that case, he said, the test would be more a statement about Russia's intention to further develop its nuclear weapons and about their growing importance in its defence posture at a time when its conventional forces have struggled in Ukraine.
Putin last week highlighted two nuclear-capable weapons, the nuclear-propelled Burevestnik cruise missile and the Sarmat intercontinental ballistic missile, which he said Russia would now mass-produce and place on combat duty.
Since the start of the war, Putin has served the West with repeated, pointed reminders of Russia's nuclear might, including by announcing the stationing of tactical nuclear weapons in Belarus. He has also suspended Moscow's participation in the New START treaty that limits the number of nuclear warheads that Russia and the United States can deploy. His comments last week on the CTBT came in direct response to a question by a security analyst, Sergei Karaganov, who suggested to Putin that Russia should lower its threshold for nuclear use in order to "sober up" the West.
Putin said he saw no need to change the doctrine which says Russia may use nuclear weapons in response to a nuclear attack or a threat to the existence of the state. But he was ambivalent on the question of nuclear testing.
"As a rule, experts say, with a new weapon you need to make sure that the special warhead will work without failures and you need to conduct tests," Putin said. He added that he was "not ready to say" whether tests were required.
RUSI's Harries said the next things to watch would be whether Russia would lay further groundwork for a test, perhaps by accusing Washington of preparing one, and whether it continued to support the CTBT by maintaining its share - as the United States and China do - of the global monitoring stations that track test-related seismic activity and radiation.
Acton said that if Russia carried out a test, the United States was likely to follow, and that could prompt China, India and Pakistan could do the same. CNN reported last month that Russia, the United States and China have all built new facilities and dug new tunnels at their nuclear test sites in recent years, according to satellite imagery.
"The more countries test, the more are then likely to test, so I'm pretty concerned about this," Acton said. "If we're in a world in which testing is going on, the first thing that shows us is that nuclear risks have risen."