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 commented that 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 he stated that the process will begin immediately. His post acknowledged the tremendous destructive power of nuclear weapons, but asserted that he had no choice but to update and renovate the US arsenal during his first term.

Experts have noted that the US would take at least 36 months to restart underground nuclear tests at the former Nevada test site, the last nuclear test occurring on September 23, 1992.

Daryl G Kimball, executive director of the ACA, criticized Trump's announcement, stating that there is no justification for resuming nuclear explosive testing and warning that it could provoke a reaction from NATO allies and other nuclear-armed countries.

Overall, Trump's move comes just months before the expiration of the New Strategic Arms Reduction Treaty with Russia, which currently limits each country to 1,550 deployed nuclear warheads.

As nations worldwide continue to grapple with the implications of nuclear arms, Trump's directive may reshape the conversations around nuclear policy and international security in the coming years.