Washington DC - The US is ready to carry out low-yield nuclear tests, ending a decades-old moratorium, a senior official said Tuesday, as he doubled down on allegations of secret explosions by China.

New START, the last treaty between the US and Russia that limited the deployment of nuclear warheads, expired this month as US President Donald Trump called for a new agreement that also includes China.

Christopher Yeaw, the assistant secretary of state for arms control and nonproliferation, indicated that Trump was serious when he said in October, without details, that the US would resume nuclear testing.

"As the president has said, the United States will return to testing on a – quote – 'equal basis,'" Yeaw said at the Hudson Institute think tank.

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"But equal basis doesn't mean we're going back to Ivy Mike-style atmospheric testing in the multi-megaton range, as some arms control folks would have you believe, hyperventilating about this issue," he said, referring to the massive 1952 thermonuclear detonation in the South Pacific.

"Equal basis, however, presumes a response to a prior standard. Look no further than China or Russia for that standard," he said.

He did not announce timing for a test, saying Trump would make a decision, but said it would be at a "level playing field."

"We're not going remain at an intolerable disadvantage," he said.

Another senior US official, at a UN meeting in Geneva as New START expired, accused China of carrying out a low-yield nuclear test in 2020 and of preparing more explosions with larger yields.

China said the allegations were "outright lies" and a pretext for the US to resume nuclear testing.

The State Department in 2024 also accused Russia of carrying out low-yield tests.

Yeaw stood by the charges on China, saying data gathered in nearby Kazakhstan on June 22, 2020, at 09:18 GMT showed a 2.75-magnitude explosion.

"There is very little possibility, I would say, that it is anything but an explosion, a singular explosion," he said, dismissing the possibility of an earthquake or mining incident.

The US last test-detonated a nuclear bomb in 1992. It has since continued with "subcritical" experiments meant to ensure safety without reaching the level to trigger a nuclear chain reaction.

China's nuclear arsenal remains far smaller than those of Russia and the US but it has been growing quickly.