President Donald Trump has ordered the US military to immediately resume testing nuclear weapons after a gap of 33 years, in what appeared to be a message to rival nuclear powers China and Russia.

Trump made the surprise announcement on Truth Social while aboard his Marine One helicopter, flying to meet Chinese President Xi Jinping for a trade-negotiating session in Busan, South Korea.

He said he was instructing the Pentagon to test the US nuclear arsenal on an "equal basis" with other nuclear powers.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis. That process will begin immediately," Trump posted.

"Russia is second, and China is a distant third, but will be even within 5 years."

Xi has more

See Full Page