The United States will closely monitor what Moscow actually does in response to Russia’s foolish decision to suspend participation in a nuclear weapons limitation deal, according to U.S. Secretary of State Antony Blinken on Tuesday.

Blinken told reporters in Athens that Russia’s announcement to halt participation was “very sad and irresponsible.” “We’ll be closely monitoring what Russia actually does. Naturally, we’ll make sure that in any situation, we are prepared for the security of both our own nation and that of our friends.”

President Vladimir Putin on Tuesday delivered a nuclear warning to the West over Ukraine, suspending a bilateral nuclear arms control treaty, announcing new strategic systems had been put on combat duty and warning that Moscow could resume nuclear tests.

Putin said that Russia was suspending participation in the New START Treaty, the last major arms control treaty between Moscow and Washington. It limits the number of nuclear warheads the world’s two biggest nuclear powers can deploy and is due to expire in 2026.

Blinken left the door open to resuming negotiations at any time.

“We remain ready to talk about strategic arms limitations at any time with Russia, irrespective of anything else going on, in the world or in our relationship,” Blinken said.

The world expects the two largest nuclear powers to act responsibly, he added.