US President Donald Trump and Russian President Vladimir Putin have spent weeks trading threats to restart nuclear tests, an escalation kicked off by Putin’s boasts about tests of the Poseidon nuclear powered missile in late October.

Days later Trump declared that he had ordered the first US nuclear test in three decades.

The leaders’ provocative public statements, including a Russian reply that it too would explore restarting nuclear weapons testing, have raised global concerns about a new nuclear arms race. Behind the scenes, both sides are also eyeing a critical upcoming deadline: February 4, when the singular remaining nuclear arms control agreement between the nations is set to expire.

The New START treaty limits both countries to a maximum of 1,550 deployed long-range nuclear warheads on delivery systems, including intercontinental ballistic missiles, submarine-launched ballistic missiles, and bombers. After a five-year extension signed by President Joe Biden, the treaty is set to expire early next year with little indication of momentum towards a new agreement.

Trump’s National Security Council called a meeting with nuclear weapons experts during the lead-up to summit between Trump and Putin in August, according to four sources familiar with the meeting.

The wide-ranging discussion included the potential