President Donald Trump has called on US military leaders to resume testing US nuclear weapons in order to keep pace with other countries such as Russia and China.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis," he wrote on social media just before meeting with Chinese President Xi Jinping in South Korea.

The US has more nuclear weapons than any other country, Trump said, with Russia second and China a "distant third". It has not conducted nuclear weapons testing since 1992.

It comes just days after Trump denounced Russia for testing a nuclear-powered missile, which reportedly has an unlimited range.

Later, on Air Force One after the two leaders' meeting, Trump said the nuclear test sites would be determined later.

"With others doing testing, I think it's appropriate that we do also," Trump said on his way back to Washington.

No country except North Korea has conducted a nuclear test explosion in this century, according to the Arms Control Association (ACA).

Trump's announcement did not include details of how the tests would occur, but wrote the "process will begin immediately".

His post on Wednesday night acknowledged the "tremendous destructive power" of nuclear weapons, but said he had "no choice" but to update and renovate the US arsenal during his first term in office.

He also said that China's nuclear programme "will be even within 5 years".

The announcement marks an apparent reversal of a long-standing US policy. The last US nuclear weapons test was in 1992, before former Republican President George HW Bush issued a moratorium as the Cold War ended.

Russia announced over the weekend that it had successfully tested two new weapons capable of carrying nuclear warheads.

These included a missile which the Kremlin said could penetrate US defence systems, and an underwater drone called Poseidon, capable of hitting the American west coast and triggering radioactive ocean swells.

But those tests did not involve the detonation of nuclear weapons.

Trump has said the US has more nuclear weapons than any other country.

The exact number of warheads held by each country is kept secret in each case - but Russia is thought to have a total of about 5,459 warheads while the US has about 5,177, according to the Federation of American Scientists (FAS).

The US-based ACA gives slightly higher estimates, saying America's nuclear stockpile sits at about 5,225 warheads, while Russia has approximately 5,580.

China is the third largest nuclear power with about 600 warheads, France has 290, the United Kingdom 225, India 180, Pakistan 170, Israel 90 and North Korea 50, FAS reports.

According to US think tank Center for Strategic and International Studies (CSIS), China has roughly doubled its nuclear arsenal in the past five years and is expected to exceed 1,000 weapons by 2030.

Trump's statement about nuclear testing came about 100 days before the expiration of the New Strategic Arms Reduction Treaty (New Start) in February 2026 - the last remaining nuclear arms treaty between the US and Russia.

The agreement limits each country to 1,550 warheads on deployed missiles capable of crossing continents.

The last time the US tested a nuclear bomb was 23 September 1992. The test took place at an underground facility in the western state of Nevada.

The project, code named Divider, was the 1,054th nuclear weapons test conducted by the US, according to the Los Alamos National Laboratory, which played a central role in helping develop the world's first atomic bomb.

The Nevada Test Site, 65 miles (105km) north of Las Vegas, is still operated by the US government.

"If deemed necessary, the site could be authorised again for nuclear weapons testing," according to the National Museum of Nuclear Science and History, which is an affiliate of the Smithsonian Institution.

But some experts note that it would take the US at least 36 months to restart underground nuclear tests at the former Nevada test site.

"Trump is misinformed and out of touch," Daryl G Kimball, executive director of the ACA, wrote on X. "The US has no technical, military, or political justification for resuming nuclear explosive testing for the first time since 1992."

"Trump will trigger strong public opposition in Nevada, from all US allies, and it could trigger a chain reaction of nuclear testing by US adversaries, and blow apart the nuclear Nonproliferation Treaty, " Mr Kimball added.

Trump's announcement also drew negative reactions from a few opposition Democrats. Representative Dina Titus, from Nevada, wrote on X: "I'll be introducing legislation to put a stop to this."

The US first marched into the nuclear era with the Trinity test of the first atomic bomb in July 1945 in the desert at Alamogordo, New Mexico.

It later became the only country in the world to use nuclear weapons in warfare after dropping two atomic bombs on the Japanese cities of Hiroshima and Nagasaki in August of the same year during World War Two.