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.

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 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.