US President Donald Trump has ordered the Pentagon to resume nuclear weapons testing, saying the move is needed to keep pace with China and Russia.

The announcement came just minutes before Trump opened a high-stakes summit with Chinese President Xi Jinping in South Korea, their first face-to-face meeting since the start of Trump’s second term.

“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 in a social media post on Thursday.

It was not immediately clear whether he was referring to testing actual nuclear warheads, which was last conducted by the United States in 1992, or testing delivery systems capable of carrying them.

The decision follows Russian President Vladimir Putin’s claim a day earlier that Moscow had successfully tested a nuclear-powered underwater drone, in defiance of US warnings.

Trump said the move was necessary to maintain parity with other nuclear powers. “We don’t do testing… we’ve halted it years, many years ago. To start again is appropriate because others are testing.” he told reporters aboard Air Force One.

He added that the United States remains open to discussions on disarmament. “I’d like to see denuclearisation… denuclearisation would be a tremendous thing,” he said, noting that Washington is already in talks with Russia and may include China in future discussions.

Trump also boasted that the United States holds “more nuclear weapons than any other country,” describing his administration’s efforts as “a complete update and renovation of existing weapons.”

“Russia is second, and China is a distant third, but will be even within five years,” he said.

Since 1996, the United States has been a signatory to the Comprehensive Nuclear-Test-Ban Treaty, which prohibits all atomic test explosions for military or civilian purposes.

China’s Foreign Ministry responded swiftly, urging Washington to “earnestly abide by the obligations of the comprehensive nuclear-test-ban treaty” and uphold its commitment to global disarmament.

“China hopes the United States will take concrete actions to safeguard the global nuclear disarmament process,” ministry spokesman Guo Jiakun said.

The United States conducted 1,054 nuclear tests between 1945 and 1992, including the two atomic bombings of Japan during World War II. The last test — a 20-kiloton underground explosion — took place in September 1992 at the Nevada Nuclear Security Site.

Then-President George H. W. Bush imposed a moratorium the following month, which subsequent administrations have maintained. Since then, the US has relied on subcritical experiments and advanced computer simulations instead of live nuclear detonations.

Putin, meanwhile, has continued to tout Russia’s nuclear advances. In televised remarks from a military hospital on Wednesday, he announced the successful testing of “Poseidon,” a nuclear-powered underwater drone he said was “impossible to intercept.”

He claimed the unmanned torpedo could travel faster than submarines, dive to extreme depths, and reach any continent.

After an earlier missile test on Sunday, Trump criticized Putin, saying the Russian leader should “end the war in Ukraine instead of testing missiles.”

A planned Trump-Putin summit in Budapest last week was called off.

While Trump did not specify when or where the new US tests would take place, he said they would “begin immediately,” a declaration that could signal a major shift in decades of nuclear restraint and arms control policy.