US President Donald Trump on Thursday said he has instructed the Department of Defence to immediately resume testing nuclear weapons on an "equal basis" with other nuclear powers.

"Because of other countries testing programmes, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis. That process will begin immediately," Trump said on Truth Social, minutes ahead of his meeting with President Xi Jinping in South Korea.

"Russia is second, and China is a distant third, but will be even within five years," Trump noted.

President Vladimir Putin said on Wednesday Russia had successfully tested a Poseidon nuclear-powered super torpedo that military analysts say is capable of devastating coastal regions by triggering vast radioactive ocean swells.

As Trump has toughened both his rhetoric and his stance on Russia, Putin has publicly flexed his nuclear muscles with the test of a new Burevestnik cruise missile on October 21 and nuclear launch drills on October 22.

The United States last tested a nuclear weapon in 1992.

Tests provide evidence of what any new nuclear weapon will do – and whether older weapons still work.

The United States opened the nuclear era in July 1945 with the test of a 20-kiloton atomic bomb at Alamogordo, New Mexico, and then dropped atomic bombs on the Japanese cities of Hiroshima and Nagasaki in August 1945 to end World War II. (Reuters)