Alleged Chinese nuclear test: US cites Kazakhstan seismic data; CTBTO says evidence inconclusive

The United States has released fresh details backing its allegation of an alleged Chinese nuclear test, pointing to seismic data recorded in Central Asia from an event near China’s Lop Nor test site on June 22, 2020. China has rejected the claim as “entirely unfounded,” while the nuclear-test monitoring body CTBTO says available data is insufficient on its own to confirm a nuclear blast.

What happened

At a Hudson Institute event in Washington on February 17, Assistant Secretary of State Christopher Yeaw said a remote seismic station in Kazakhstan registered what he described as an “explosion” (magnitude around 2.75) about 450 miles (720 km) from China’s Lop Nor test grounds, dated June 22, 2020. Yeaw said the signal was not consistent with an earthquake or mining blasts and was “what you would expect with a nuclear explosive test.”

China’s embassy in Washington denied the allegation, saying it was aimed at creating a pretext for the US to resume nuclear testing and accusing Washington of political manipulation.

Key details

• US claim: The seismic signal from June 22, 2020 is consistent with a “singular explosion” near Lop Nor; Yeaw also alleged China may have used “decoupling” (detonating inside a large underground cavity) to reduce detectable shockwaves.
  
  

• CTBTO response: CTBTO Executive Secretary Robert Floyd said the International Monitoring System detected two very small seismic events 12 seconds apart at 9:18am UTC on June 22, 2020, in the vicinity of specified coordinates—and noted these signals were below the system’s approximate detection capability for identifying events consistent with nuclear tests at ~500 tonnes of TNT equivalent or higher. With this data alone, CTBTO said it is not possible to assess the cause with confidence.
  
  

• China’s position: China’s foreign ministry has pointed to CTBTO’s stance to argue the US accusation is groundless and reiterated support for the CTBT.

  
  

Context and why it matters

The dispute is unfolding amid a wider arms-control reset after New START expired on February 5, 2026, leaving the US and Russia without binding limits on deployed strategic warheads and delivery systems for the first time in decades. Washington has pressed Beijing to join talks on a replacement framework, while China has argued its arsenal is much smaller than those of the US and Russia.

Earlier this month, the US publicly raised the allegation in Geneva, where Under Secretary Thomas DiNanno said the US was aware of Chinese “yield-producing” nuclear explosive tests and again referenced “decoupling.” CTBTO’s public position at that time was that its system did not detect an event consistent with a nuclear weapon test explosion, and subsequent analyses did not change that determination.

What happens next

• Verification gap remains: CTBTO has stressed that treaty-based verification tools to address disputed claims or smaller explosions can only be used once the Comprehensive Nuclear-Test-Ban Treaty (CTBT) enters into force.

• Diplomatic fallout likely: The allegation is expected to feed into US-China arms-control messaging and broader debates on whether Washington should adjust its nuclear-testing posture—something Beijing says the US is attempting to justify.