The world has lurched a step closer to the prospect of nuclear war, say researchers, after Russia declared last month that it would suspend its participation in its last major nuclear-arms treaty with the United States.

Russian President Vladimir Putin’s announcement will formally end inspections under an agreement known as the New START treaty (Measures for the Further Reduction and Limitation of Strategic Offensive Arms).

The inspection programme had allowed scientists on both sides to verify that the other was complying with the agreement. The move comes amid soaring tensions between Russia and the West over the war in Ukraine.

New START limits Russia and the United States to 1,550 strategic warheads each, but has been facing challenges for years. The COVID-19 pandemic forced a pause in weapons inspections, and they have not restarted since Russia invaded Ukraine last year.

US US government officials said in January that Russia was in violation of the treaty because it would not allow inspections to resume. However, Russia’s official suspension of the inspections is another blow to international efforts to control nuclear weapons, say experts, who worry that the world’s largest nuclear powers will not be subject to any arms-control obligations once New START expires in three years — or even sooner.