Donald Trump’s statement last week that the US would resume testing nuclear weapons “on an equal basis” with Russia and China wrongfooted his own officials as much as it did Beijing and Moscow. Restarting warhead tests would break a three-decade moratorium by the major nuclear powers. Coming after Russia’s Vladimir Putin bragged of two new weapons delivery systems, it bolstered concerns that the world is sliding into a new nuclear arms race — when much of the cold war-era arms control architecture has collapsed.

Trump’s comments appeared primarily a response to Putin’s claims to have tested two nuclear-powered delivery platforms: Burevestnik, a long-range cruise missile, and Poseidon, a torpedo said to be able to devastate coastal regions with a radioactive tidal wave. Russia’s president said both could evade existing defences, in a swipe at Trump’s plan to expand today’s US missile defence system into an elaborate “Golden Dome”.

Trump may have intended to signal the US would step up testing of delivery systems, not warheads. In a weekend TV interview, though, he appeared to confirm that he meant explosive nuclear testing. His energy secretary said the US would simply continue systems tests involving “non-nuclear explosions”.