Assistant Secretary of State Christopher Yeaw told an event at the Hudson Institute think tank in Washington that a remote seismic station in Kazakhstan measured an “explosion” of magnitude 2.75 located 450 miles away at the Lop Nor test grounds in western China on June 22, 2020.

“I’ve looked at additional data since then. There is very little possibility, I would say that it is anything but an explosion, a singular explosion,” said Yeaw, adding that the data were not consistent with mining blasts.

“It’s also entirely not consistent with an earthquake,” said Yeaw, a former intelligence analyst and defence official who holds a doctorate in nuclear engineering.

“It is … what you would expect with a nuclear explosive test.”

The Comprehensive Test Ban Treaty Organisation, which is charged with detecting nuclear test explosions, said that there was insufficient data to confirm Yeaw’s allegation with confidence.

A spokesperson for the Chinese embassy in Washington said the allegation about China conducting a nuclear test was “entirely unfounded” and an attempt “to fabricate excuses for resuming” US nuclear testing.

“This is political manipulation aimed at pursuing nuclear hegemony and evading its own nuclear disarmament responsibilities,” Chinese embassy spokesperson Liu Pengyu said in an emailed statement.

“China urges the US to reaffirm the five nuclear-weapon states’ commitment to refraining from nuclear tests, uphold the global consensus against nuclear tests, and take concrete steps to safeguard the international nuclear disarmament and non-proliferation regime,” Liu added.

US President Donald Trump is pressing China to join the US and Russia in negotiating a replacement pact to New START, the last US-Russia strategic nuclear arms limitation agreement, which expired on February 5.

The treaty’s expiration has fueled concerns that the world is on the verge of an accelerated nuclear arms race.

China denies nuclear testing

China, which has signed but not ratified the 1996 international treaty banning nuclear testing, denied setting off an underground nuclear test blast after the US first levelled the allegation at an international conference earlier this month.

China’s last official underground test took place in 1996.

The PS23 seismic station in Kazakhstan is part of a globe-spanning monitoring system operated by the Comprehensive Test Ban Treaty Organisation (CTBTO).

The organisation’s executive secretary, Robert Floyd, said in a statement that the PS23 station recorded “two very small seismic events” spaced 12 seconds apart on June 22, 2020.

The CTBTO’s monitoring system can detect “events” consistent with nuclear test explosions with yields of 551 tons (500 metric tons) of TNT or greater, he said.

“These two events were far below that level. As a result, with this data alone, it is not possible to assess the cause of these events with confidence,” Floyd said.

Yeaw said that China tried to conceal the test by using a method known as decoupling, in which the device is detonated inside a large underground chamber to reduce the magnitude of the shockwaves it sends through the surrounding rock.

Like China, the US has signed but not ratified the test ban.

Under international law, both countries are obligated to uphold the pact.

The US conducted its last underground nuclear test in 1992 and has been relying on a multibillion-dollar programme that uses advanced tools and supercomputer simulations to ensure its nuclear warheads work properly.

China has rejected Trump’s call to negotiate a three-way treaty to replace New START, contending that its strategic nuclear arsenal is dwarfed by those of Washington and Moscow, the world’s largest nuclear powers.

The Pentagon says China now has more than 600 operational warheads and is conducting a major expansion of its strategic nuclear force.

It projects that China will field more than 1,000 warheads by 2030.