China denied testing nuclear weapons on Monday after US President Donald Trump alleged the country was among those to have stepped up tests away from public view.

"China has always adhered to the path of peaceful development, pursued a policy of no first use of nuclear weapons, upheld a self-defence nuclear strategy and abided by its commitment to suspend nuclear testing," said China foreign ministry spokeswoman Mao Ning.

Mao also urged the US to uphold the moratorium on nuclear testing, following Trump’s surprise announcement on Thursday that he had ordered his “war department” to “immediately” resume nuclear weapons tests to keep up with the alleged Russian and Chinese build-up.

China hopes the US will “take concrete actions to safeguard the international nuclear disarmament and non-proliferation regime and maintain global strategic balance and stability”, Mao continued.

It remains unclear if the US would actually detonate a nuclear weapon or simply test nuclear weapons delivery capabilities.

The US energy secretary has downplayed the suggestion that would mean a nuclear explosion.

“I think the tests we’re talking about right now are system tests. These are not nuclear explosions,” Chris Wright said in a Fox News interview.

Earlier, Trump made the claims about “global secret nuclear tests”, without offering evidence, in a television interview with CBS.

“Russia’s testing, and China’s testing, but they don’t talk about it,” he said.

“I don’t want to be the only country that doesn’t test,” he continued, adding North Korea and Pakistan to the list of nations allegedly testing arsenals.

The US has not set off a nuclear explosion since 1992. No country other than North Korea is known to have conducted a nuclear detonation for decades. Russia and China report they have not carried out such tests since 1990 and 1996, respectively.

However, Russia said last week it had tested a new nuclear-powered cruise missile, the Burevestnik, and a nuclear-powered and nuclear-capable underwater drone.