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.

Trump's post on Wednesday night acknowledges the tremendous destructive power of nuclear weapons, but he stated he had no choice but to update and renovate the US arsenal during his first term in office.

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

While details of the tests were not disclosed, Trump insisted that the process will begin immediately. This marks a reversal of a long-standing US policy, as the last nuclear weapons test was conducted in 1992, following a moratorium issued by former Republican President George HW Bush as the Cold War ended.

The US test site in Nevada, which is still operated by the US government, could potentially be authorized for nuclear weapons testing again if deemed necessary.

Previously, the last nuclear bomb test took place on September 23, 1992, at an underground facility in Nevada, conducted as Project Divider, the 1,054th nuclear test by the US.