U.S. President Donald Trump on Thursday directed the Pentagon to restart nuclear weapons testing "on an equal basis," citing testing programs by other countries.

Speaking to reporters aboard Air Force One after meeting Chinese President Xi Jinping in South Korea, Trump said the decision "had to do with others," without naming specific countries.

"They seem to all be nuclear testing. We have more nuclear weapons than anybody. We don't do testing ... But with others doing testing, I think it's appropriate that we do also," he said, adding that details on the planned tests would be announced later.

Before his meeting with Xi, Trump wrote on Truth Social that "because of other countries testing programs, I have instructed the Department of War to start testing our Nuclear Weapons on an equal basis. That process will begin immediately."

It was not immediately clear what Trump meant by "equal basis." The Department of Defense was rebranded the Department of War on Sept. 5 under an executive order, though Congress has not approved the change.

The announcement followed reports that Russia had successfully tested a Poseidon nuclear-powered super torpedo on Wednesday.

"For the first time, we managed not only to launch it with a launch engine from a carrier submarine, but also to launch the nuclear power unit on which this device passed a certain amount of time," Russian President Vladimir Putin was quoted as saying.

Moscow also tested its new Burevestnik nuclear cruise missile on Oct. 21 and conducted nuclear launch drills a day later.

The U.S. has not conducted a live nuclear weapons test since 1992, when then-President George H.W. Bush imposed a unilateral testing moratorium.

In 1996, the Comprehensive Nuclear Test Ban Treaty was signed by then-President Bill Clinton.

Trump also claimed in his Truth Social post that the U.S. has the world's largest nuclear arsenal, adding that Russia and China would match its nuclear weapon stockpile within five years — a statement contradicted by independent data.

According to estimates by the Arms Control Association, the International Campaign to Abolish Nuclear Weapons and the Federation of American Scientists, Russia possesses the most nuclear weapons in the world.

Russia is estimated to have 5,459 nuclear weapons in 2025, compared to 5,177 for the U.S. and 600 for China, according to the Federation of American Scientists. The nonprofit was founded in 1945 by atomic scientists after World War II to advocate for international oversight of nuclear energy.