You'll find out soon: Trump dodges question on underground nuclear tests

US President Donald Trump

On Friday, US President Donald Trump reiterated that the United States would move forward with restarting nuclear testing but sidestepped a direct response on whether this would include underground nuclear tests like those conducted 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, when he was questioned over underground nuclear testing.

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

Trump on Thursday said he had instructed the US military to resume nuclear weapons testing after a 33-year hiatus -- a decision widely seen as a signal to nuclear rivals China and Russia.

He made the unexpected announcement on social media while flying aboard Marine One to Busan, South Korea, for trade talks with Chinese President Xi Jinping.

Trump said the US has "more nuclear weapons than any other country", crediting his first term in office for what he described as a "complete update and renovation" of the nation's nuclear arsenal.

"The United States has more Nuclear Weapons than any other country. This was accomplished, including a complete update and renovation of existing weapons, during my First Term in office," Trump wrote.

"Because of the tremendous destructive power, I HATED to do it, but had no choice! Russia is second, and China is a distant third, but will be even within 5 years," he added.

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.