• The Trump administration is quietly engaging China over allegations of a concealed low-yield nuclear test in 2020 and concerns about Beijing's expanding arsenal.
  • Talks focus on risk reduction, verification, and transparency, with another round expected next month.
  • The dialogue unfolds after the expiration of New START, leaving no binding caps on U.S. and Russian strategic forces.

Fragile Diplomacy

U.S. and Chinese officials have met on the margins of multilateral gatherings in Geneva, New York, and Casablanca in recent months, according to people familiar with the matter, as Washington presses Beijing over allegations that it conducted a secret low-yield nuclear test in 2020 and is rapidly expanding its nuclear forces. The discussions, which remain fragile and are focused on reducing nuclear risks rather than establishing formal arms-control limits, are expected to continue next month, the people said.

The talks center on U.S. claims that China may have violated the 1996 Comprehensive Nuclear-Test-Ban Treaty (CTBT) with a covert test at its Lop Nur site. According to U.S. officials, a Kazakhstan-based monitoring station registered a magnitude-2.75 seismic event on June 22, 2020, which they assess was a single underground explosion rather than an earthquake or conventional mining activity. They contend China may have used a technique known as "decoupling"—detonating an explosive in a large underground cavity to muffle the seismic signature.

Beijing categorically denies the allegation. Chinese officials have called the accusation unfounded and argue that Washington is manufacturing a rationale to resume its own testing. China says it supports the global norm against nuclear testing and has observed a de facto moratorium since its last acknowledged test in 1996.

The CTBTO, which operates the international monitoring network associated with the CTBT, recorded two very small events 12 seconds apart but says the available data cannot establish their cause with confidence. Its global system is designed to reliably detect explosions around 500 tons of TNT equivalent or higher; the alleged event was well below that threshold. The CTBTO's nonconfirmation does not prove that no test occurred, but it means the publicly described data do not independently settle the allegation.

No Breakthrough

Efforts to bridge the divide have hit a snag, as the two sides remain far apart on the fundamental question of whether a test took place. Washington is seeking stronger verification measures, greater transparency, and more advance notice of ballistic-missile tests. Beijing has resisted these proposals, viewing them as one-sided and politically motivated.

"What institutional investors like us are really focused on is regulatory stability," said one person familiar with the U.S. position, speaking on condition of anonymity. "But in this context, it's about strategic stability, and that requires verification."

The discussions are taking place against the backdrop of China's ongoing nuclear modernization. According to SIPRI estimates, China's stockpile rose from roughly 600 warheads in January 2025 to about 620 in January 2026. While this remains far below the total U.S. and Russian inventories, China is expanding and modernizing more rapidly than other nuclear-armed states. The U.S. government projects more than 1,000 Chinese warheads by 2030.

The talks also follow the expiration of New START on February 5, 2026, which removed legally binding caps of 1,550 deployed strategic warheads and 700 delivery systems on the U.S. and Russia. The Trump administration argues that a replacement arrangement must include China; Beijing has resisted trilateral limits, maintaining that its much smaller arsenal makes it inappropriate to treat China on the same footing as the U.S. and Russia.

Market and Strategic Fallout

While not a corporate headline, the geopolitical implications are already rippling through markets. Defense spending could rise as the U.S. and China deepen their nuclear competition, reinforcing demand for missile defense, early-warning systems, satellite sensing, and cyber resilience. Nuclear tensions also add to strategic-risk premiums already affecting cross-border technology, semiconductors, critical minerals, and shipping.

A collapse of testing restraints or arms-control dialogue could worsen tail risks for global markets, particularly amid simultaneous disputes over Taiwan, trade, and regional military activity.

U.S. allies in Asia and Europe—including Japan, South Korea, Australia, and NATO governments—have a strong interest in clear deterrence commitments and credible early warning. China's neighbors, such as India and Southeast Asian states, face greater uncertainty as Beijing develops more survivable land-, sea-, and potentially air-based nuclear capabilities.

Arms-control advocates worry that the allegations could weaken the longstanding international no-testing norm, especially if unproven accusations become a basis for renewed testing by major powers. The episode also highlights a real limitation: extremely low-yield or deliberately muffled events are difficult to attribute with high confidence using public seismic evidence alone.

A U.S. official said the administration remains committed to dialogue but declined to comment on specific allegations. A Chinese embassy spokesperson did not respond to a request for comment.

The immediate path forward remains uncertain. The most constructive near-term outcome would be a continuing channel for discussion and practical confidence-building measures, such as prior notification for some ballistic-missile launches or expert-level exchanges on seismic monitoring. The more negative possibility is hardened rhetoric, with China rejecting transparency initiatives as one-sided and the U.S. using the unresolved allegation to justify a tougher deterrence posture.

Without a deal, the risk of miscalculation grows. As one former U.S. official put it, "The facts remain contested, but the allegation has become leverage and urgency for opening a nuclear-risk dialogue at a moment when the last major U.S.–Russia nuclear treaty has expired."

Correction: An earlier version of this article misstated the date of the alleged test. It occurred on June 22, 2020, not June 22, 2019.