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.

This announcement 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).

His post acknowledged the tremendous destructive power of nuclear weapons, but he stated he had no choice but to update and renovate the US arsenal during his presidency.

Trump's announcement marks an apparent reversal of a long-standing US policy. The last US nuclear weapons test was in 1992, under the administration of former President George HW Bush.

The decision comes amid rising tensions and advancements in other nuclear programs, with Russia recently testing two new weapons capable of carrying nuclear warheads.

Concerns have arisen about the implications of this potential resumption of testing, including negative reactions from both domestic and international leaders, as many experts argue there is no justification for renewing such tests.