Washington DC - President Donald Trump claimed Sunday that countries including Russia and China have conducted underground nuclear tests unknown to the public, and that the US would follow suit.

"Russia's testing, and China's testing, but they don't talk about it," he told CBS' 60 Minutes in an interview released Sunday.

"I don't want to be the only country that doesn't test," Trump said, adding North Korea and Pakistan to the list of nations allegedly testing their arsenals.

Confusion has surrounded Trump's order that the US begin testing, particularly if he meant conducting the country's first nuclear explosion since 1992.

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The Republican first made his surprise announcement on social media Thursday, minutes before entering a summit with Chinese leader Xi Jinping in South Korea.

The announcement came in the wake of Russia saying it had tested a new nuclear-powered cruise missile, the Burevestnik, and a nuclear-powered and nuclear-capable underwater drone.

Asked directly if he planned for the US to detonate a nuclear weapon for the first time in more than three decades, Trump told CBS: "I'm saying that we're going to test nuclear weapons like other countries do, yes."

No country other than North Korea is known to have conducted a nuclear detonation for decades.

Russia and China have not carried out such tests since 1990 and 1996 respectively.

Pressed on the topic, Trump said: "They don't go and tell you about it."

"As powerful as they are, this is a big world. You don't necessarily know where they're testing."

"They test way underground where people don't know exactly what's happening with the test. You feel a little bit of a vibration," he added.

Trump's energy secretary on Sunday however ruled out plans to set off 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 on Sunday.

"These are what we call 'non-critical explosions,' so you're testing all the other parts of a nuclear weapon to make sure they deliver the appropriate geometry and they set up the nuclear explosion," he said.

The US has been a signatory since 1996 to the Comprehensive Nuclear-Test-Ban Treaty, which bans all atomic test explosions, whether for military or civilian purposes.