Does Trump's nuclear testing raise the stakes - or are we already in an arms race?

President Donald Trump has announced the US will start testing nuclear weapons in what could be a radical shift in his nation's policy.

"Because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis," Trump wrote on his social media platform, Truth Social, as he was about to meet the Chinese president on Thursday.

"That process will begin immediately."

The world's nuclear-armed states - those acknowledged as belonging to the so-called nuclear club and those whose status is more ambiguous - regularly test their nuclear weapons' delivery systems, such as a missile that would carry a nuclear warhead.

Only North Korea has actually tested a nuclear weapon since the 1990s - and it has not done so since 2017.