In the middle of a high-stakes diplomatic tour of Asia, President Donald Trump threatened on social media to resume nuclear testing for the first time in more than 30 years.

He made the threat just minutes before he was scheduled to meet President Xi Jinping of China, who is overseeing one of the fastest buildups of a nuclear arsenal on earth.

“Because of other countries’ testing programmes, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis,” Trump wrote on Truth Social, his social media site, saying the process would begin immediately.

The words “on an equal basis” may mean he will show off the power of American missiles or undersea nuclear assets, rather than detonate a nuclear weapon. The US routinely tests unarmed missiles. Trump did not clarify his remarks to reporters while greeting Xi.

While China is rapidly expanding its nuclear stockpile, and deploying missiles in new silos, it has not tested a nuclear weapon since 1996. Russia has not conducted a confirmed test since 1990. And while the United States has never ratified the Comprehensive Nuclear Test Ban Treaty, which bans weapon detonations, past Presidents have largely observed its provisions.

It was not clear what prompted the announcement, which Trump appeared to have made while in Marine One, the presidential helicopter, as he was flying to meet Xi. But he may well have been angered by recent tests of exotic nuclear delivery systems by Russia.

In the past few days, President Vladimir V. Putin has said Russia had successfully tested a nuclear-powered and nuclear-capable cruise missile, and separately, a nuclear torpedo called the Poseidon. The torpedo is designed to travel under the Pacific from Russia’s east to hit the American West Coast.

Those systems were known to observers; Putin showed them off during Trump’s first term. It is unclear whether they are fully operational. Putin’s sabre rattling, in the wake of fizzled plans for a summit meeting with Trump about Ukraine, consisted of tests of the delivery vehicles; he did not detonate any nuclear weapons.

The Kremlin said on Thursday that Russia’s test of the nuclear-powered missile and nuclear-powered torpedo were not nuclear weapons tests. Putin, Kremlin spokesman Dmitry Peskov said, had cautioned that if any country tested a nuclear weapon, then Russia would too.

For years, American nuclear weapons engineers have said that more nuclear testing was unnecessary, since they could model tests on a computer rather than risk the kind of detonations that were once set off in the Pacific or underground in Nevada. But in recent years, as the US has begun to modernise its ageing arsenal, there have been calls to resume testing.

Trump said he had authorised the Pentagon — what he now calls the department of war — to swiftly conduct nuclear tests. But historically, those tests were done by the energy department, which designs and builds America’s nuclear weapons.

The President’s statement came about 100 days before the expiration of the last major nuclear arms control treaty between the US and Russia, called New START. It limits each country to 1,550 deployed strategic weapons, the kind that can cross continents, usually on intercontinental ballistic missiles.

The treaty cannot be extended again. But after meeting Trump in Anchorage, Alaska, in July, Putin suggested that both countries could informally agree to observe the limits on deployed weapons for another year, presumably while they began to discuss a replacement treaty, or some other kind of arrangement to avoid resuming a Cold War-like arms race.

Trump told reporters he thought Putin’s idea was a good one. But that was before he accused the Russian leader of dragging along talks while continuing to attack Ukrainian civilians. There is no indication any talks with the Russians about a successor to the New START treaty have begun. The tests of new, exotic weapons delivery systems seemed a particular challenge because they are not covered by the treaty’s limits.

China poses a particularly difficult nuclear conundrum for Trump. It has never been a party to nuclear arms limitations treaties; during the Cold War, its “minimum deterrent” of just a few hundred nuclear weapons, compared to the thousands held by Russia and the US, seemed too small to worry about.

But Xi abandoned that decades-long policy, first secretly, then publicly as China created new missile silos in plain view of American spy satellites. The Pentagon estimates it will have 1,000 or so deployed weapons in 2030, and 1,500 in 2035. That would put it on par with the current deployed arsenals wielded by Washington and Moscow.

Until it reaches rough parity, China appears to have no interest in joining arms control talks; Trump has suggested he thought he could convince Xi to join such negotiations.

Should Trump go ahead and test a nuclear weapon, perhaps at the testing area outside of Las Vegas, it would most likely trigger similar tests by other nuclear-armed states. Among American allies, those include Britain and France and Israel, which has an undeclared arsenal of nearly 100 weapons. India, Pakistan and North Korea also possess growing nuclear stockpiles; North Korea was the last to conduct a confirmed nuclear test, during Trump’s first term.

New York Times News Service