US President Donald Trump reaffirmed on Friday that the US would resume nuclear testing, but he did not answer directly when asked whether that would include underground nuclear tests that were common during the Cold War.

"You'll find out very soon, but we're going to do some testing," Trump told reporters aboard Air Force One as he flew to Palm Beach, Florida, when asked about underground nuclear tests.

"Other countries do it. If they're (going) to do it, we're going to do it, OK?"

Trump on Thursday said he ordered the US military to immediately restart the process for testing nuclear weapons after a halt of 33 years.

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

It was not immediately clear whether Trump was referring to nuclear-explosive testing, which would be carried out by the National Nuclear Security Administration, or flight testing of nuclear-capable missiles.

During a trip to Malaysia, US Defence Secretary Pete Hegseth said resuming testing was a "very responsible way" to maintain nuclear deterrence, adding that the Pentagon would work with the Department of Energy.

"We're moving out quickly," Hegseth said.

No nuclear power - other than North Korea most recently in 2017 - has carried out explosive nuclear testing in over 25 years. (Reuters)