President Donald Trump 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.

Trump's post on Wednesday night acknowledges 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".

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

It 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.

Trump's post came just before Xi landed in South Korea for the first face-to-face meeting between the two since 2019. The post appeared as he was aboard a helicopter, Marine One, while en route to meet Xi at Gimhae International Airport.

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 authorized again for nuclear weapons testing," according to the National Museum of Nuclear Science and History, which is an affiliate of the Smithsonian Institution.