US President Donald Trump has ordered the US military to immediately resume testing nuclear weapons after a gap of 33 years, minutes before beginning a meeting with Chinese President Xi Jinping.

Trump made the surprise announcement on Truth Social while on his way to meet Xi on Thursday for a trade session in Busan, South Korea.

He said he had instructed 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."

A senior Russian MP said Trump's decision would herald a new era of un

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