US President Donald Trump ordered the US military on Thursday to immediately resume testing nuclear weapons after a gap of 33 years, minutes before beginning a meeting with Chinese President Xi Jinping.

Trump made the surprise announcement on Truth Social while he was aboard his Marine One helicopter flying to meet Xi for a trade negotiating session in Busan, South Korea. He said he was instructing the Pentagon to test the US nuclear arsenal on an "equal basis" with other nuclear powers.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis. That process will begin immediately," Trump posted. "Russia is second, and China is a distant third, but will be even within 5 years."

A senior Russian lawmaker said Trump's decision would herald a new era of unpredictability and open confrontation, state news agency RIA reported.

China's Foreign Ministry called for the US to abide by its commitment to a moratorium on nuclear testing and uphold the global strategic balance and stability.

It was not immediately clear whether Trump was referring to nuclear-explosive testing, which would be carried out by the National Nuclear Security Administration, or flight testing of nuclear-capable missiles.

No nuclear power, other than North Korea, has carried out explosive testing in more than 25 years.

NUCLEAR TESTING SITES TO BE DETERMINED LATER

Later, on his way back to Washington, Trump said testing was needed to ensure the US keeps up with its rival nuclear powers.

"With others doing testing, I think it's appropriate that we do also," Trump said on board Air Force One, adding that nuclear test sites would be determined later.

Asked whether the world was entering a more risky phase around nuclear weapons, Trump dismissed the threat, saying US stocks were "well locked up" before adding he would welcome denuclearisation.

"I'd like to see a denuclearisation because we have so many and Russia's second and China's third and China will catch up within four or five years," he said.

"We are actually talking to Russia about that and China would be added to that if we do something."

CHINA MORE THAN DOUBLED NUCLEAR ARSENAL IN LAST 5 YEARS

Trump's decision to restart nuclear weapons testing follows a rapid expansion by China of its nuclear stockpile in recent years, and comes just after Russia announced what it called a successful test of a nuclear-powered and nuclear-capable cruise missile as well as a nuclear-powered torpedo.

Trump addressed the Russian moves on his way to Asia earlier this week, telling reporters that Russian President Vladimir Putin should be working to end the war in Ukraine "instead of testing missiles."

Beijing has more than doubled the size of its arsenal to an estimated 600 nuclear weapons in 2025 from 300 weapons in 2020, according to the Center for Strategic and International Studies, a Washington-based think tank.

It said US military officials estimate that China will have more than 1,000 nuclear weapons by 2030. A Victory Day parade in September revealed five nuclear capabilities that can all reach the continental United States, CSIS said.

The Washington-based Arms Control Association says the United States has a stockpile of 5,225 nuclear warheads and Russia has 5,580.

Putin said on Wednesday Russia had successfully tested a Poseidon nuclear-powered super torpedo that military analysts say is capable of devastating coastal regions by triggering vast radioactive ocean swells.

As Trump has toughened both his rhetoric and his stance on Russia, Putin has publicly flexed his nuclear muscles with the test of a new Burevestnik cruise missile on October 21 and nuclear launch drills on October 22.

NEGATIVE REACTIONS TO TRUMP'S POST The reaction to Trump's announcement on testing was swift. Representative Dina Titus, a Democrat from Nevada, said on X: "I'll be introducing legislation to put a stop to this."

Daryl Kimball, director of the Arms Control Association, said it would take the United States at least 36 months to resume contained nuclear tests underground at the former test site in Nevada.

"Trump is misinformed and out of touch. The US has no technical, military, or political reason to resume nuclear explosive testing for the first time since 1992," Kimball said on X.

Trump's announcement could "trigger a chain reaction of nuclear testing by US adversaries, and blow apart the nuclear Non-Proliferation Treaty."

Apart from providing technical data, a US test would be seen in Russia and China as a deliberate assertion of Washington's strategic power. Putin has repeatedly said that Russia will test if the United States does.

In August, Trump said he had discussed nuclear arms control with Putin and wanted China to get involved. Beijing responded by saying it was "unreasonable and unrealistic" to ask the country to join in nuclear disarmament negotiations with the two countries, since its arsenal was much smaller.

Trump had first laid out his intention to pursue nuclear arms control efforts in February, saying he wanted to begin discussions with both Putin and Xi about imposing limits on their arsenals.

Most major nuclear powers except North Korea stopped explosive nuclear testing in the 1990s. North Korea conducted its last nuclear test in 2017. Russia's last confirmed test was in 1990, followed by the last US test in 1992, and by China's in 1996.

The United States opened the nuclear era in July 1945 with the test of a 20-kiloton atomic bomb at Alamogordo, New Mexico, and then dropped atomic bombs on the cities of Hiroshima and Nagasaki in August 1945 to force Japan to surrender in World War Two.