President Donald Trumpon Thursday ordered the US military to immediately restart the process for testing nuclear weapons after a halt of 33 years, a move that appeared to be a message to rival nuclear powers China and Russia.

Trump made the surprise announcement on Truth Social while aboard his Marine One helicopter flying to meet Chinese President Xi Jinping for a trade-negotiating session in Busan, South Korea.

“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.”

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 most recently in 2017 - has carried out explosive nuclear testing in over 25 years.

Post-Soviet Russia has never tested. The Soviet Union last tested in 1990, and China in 1996. Trump’s fellow Republican, President George H.W. Bush, announced a US testing moratorium in 1992 after the last U.S test in September that year.

Russia - which tested a new nuclear-powered cruise missile on October 21, held nuclear readiness drills on October 22 and tested a new nuclear-powered autonomous torpedo on October 28 - said it hoped Trump had been properly informed that Moscow had not tested an actual nuclear weapon itself.

“President Trump mentioned in his statement that other countries are engaged in testing nuclear weapons. Until now, we didn’t know that anyone was testing,” Kremlin spokesman Dmitry Peskov told reporters.

Putin, who commands the world’s biggest arsenal of nuclear warheads, has repeatedly said that if any country tests a nuclear weapon then Russia will do so too.

Xi Jinping has more than doubled China’s nuclear warhead arsenal to an estimated 600 nuclear weapons in the past five years, while Russian President Vladimir Putin has tested two new nuclear-powered weapons in recent days.

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.

Robert Floyd, head of the Vienna-based Comprehensive Nuclear-Test-Ban Treaty Organization, expressed alarm.

“Any explosive nuclear weapon test by any state would be harmful and destabilising for global non-proliferation efforts and for international peace and security,” he said.

U.N. Secretary-General Antonio Guterres has repeatedly said current nuclear risks are already alarmingly high and urged countries to avoid all actions that could lead to miscalculation or escalation with “catastrophic” consequences, said deputy U.N. spokesperson Farhan Haq.

“As he has said, we must never forget the disastrous legacy of over 2,000 nuclear weapons tests carried out over the last 80 years, and that nuclear testing can never be permitted under any circumstances,” said Haq.

NUCLEAR TESTING SITES TO BE DETERMINED

Later, on his way back to Washington, Trump said testing was needed to ensure the US keeps up with 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 that 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.”

US Vice President JD Vance told reporters it was important to ensure the US nuclear arsenal “actually functions properly, and that’s part of a testing regime.”

RUSSIAN TESTS

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

The United States has a stockpile of 5,225 nuclear warheads and Russia has 5,580, according to the Washington-based Arms Control Association, a nonpartisan group advocating nuclear nonproliferation agreements.

The Pentagon estimates that China will have more than 1,000 nuclear weapons by 2030.

Military analysts say the Poseidon nuclear-powered super torpedo Putin said Russia had successfully tested is capable of devastating coastal regions by triggering vast radioactive ocean swells. Both it and Burevestnik cruise missile are nuclear-capable.

US Democratic Senator Edward Markey of Massachusetts, who co-chairs the congressional Nuclear Weapons and Arms Control Working Group, said he was introducing legislation to prevent Trump from restarting nuclear testing by forbidding funding for it.

“A Trumpatomics plan would provoke Russia and China to resume nuclear testing, and China in particular has much more to gain from this than does the United States,” he said in a statement.

“This is a reckless directive from Trump that will only make the country and the world less safe and lead to a terrible new nuclear arms race.”

Daryl Kimball, director of the Arms Control Association, said it would take the US 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,” he said, warning that Trump’s announcement could “trigger a chain reaction of nuclear testing by US adversaries, and blow apart the nuclear Non-Proliferation Treaty.”

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 since its arsenal was much smaller.