President Donald Trump called on U.S. military leaders to resume testing U.S. 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 U.S. 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 acknowledges the tremendous destructive power of nuclear weapons, but he said he had no choice but to update and renovate the U.S. arsenal during his first term in office.

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

The post did not include details of how the tests would occur, but wrote the process will begin immediately.

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

The last time the U.S. tested a nuclear bomb was 23 September 1992 at an underground facility in Nevada, part of the project code-named Divider.

The Nevada Test Site, which is still operated by the U.S. government, could be authorized again for nuclear weapons testing if deemed necessary.