Shortly before meeting with Chinese President Xi Jinping in South Korea, US President Donald Trump posted on social media that he had instructed the Pentagon to start "immediately" testing nuclear weapons.

The reason, he said, was because other countries "seem to all be nuclear testing".

"We have more nuclear weapons than anybody. We don't do testing. I see them testing and I say, well, if they're going to test, I guess we have to test," he later told reporters aboard Air Force One.

The US has not conducted nuclear weapons tests detonating warheads for 33 years.

Mr Trump has not specified what type of testing he had in mind , apart from being "on equal basis" with other countries.

If the US resumes explosive nuclear weapons tests, experts warn it will be a dangerous escalation, espec

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