Russia has said it is no longer bound on the number of nuclear warheads it could deploy after the New START agreement ended Thursday, formally releasing both Moscow and Washington from a raft of restrictions.

"Maintaining dialogue between militaries is an important factor in global stability and peace, which can only be achieved through strength, and provides a means for increased transparency and de-escalation," the US military's European Command said in a statement.

The agreement to reestablish the military dialogue, suspended in 2021, also came after "productive and constructive progress" at Ukraine peace talks in Abu Dhabi attended by Trump's special envoy Steve Witkoff and son-in-law Jared Kushner, European Command said.

Campaigners have warned the nuclear weapons treaty's demise could unleash a new arms race between the world's top nuclear powers, and encourage China to expand its arsenal.

NATO urged "responsibility and restraint."