China has detained a US seismologist who tracks nuclear tests for nearly two years on espionage charges, his family says.

Chen Youlin, 54, was arrested in November 2024 during a trip to Beijing to visit family, according to hostage advocacy group Global Reach. The family decided to speak out after they saw no sign of Beijing freeing Chen.

His wife Rong Yufang, also a seismologist, said Chen worked closely with Chinese colleagues, and the allegations are "both wrong and inconsistent with the public and collaborative nature of the work that he has done".

His published work centres on North Korea, a close friend of China and long sanctioned for its nuclear weapons programme and underground tests.

It is unclear if and how Chen's work touched on Beijing's nuclear programme. US intelligence suggests that China is developing a new arsenal and has conducted secret tests, which Beijing denies.

When asked about the case at a daily press briefing on Tuesday, China's foreign ministry said its "judicial authorities handle cases in accordance with the law".

"There is no such thing as so-called wrongful detention," the ministry's spokesman Lin Jian said. In China, espionage convictions can lead to life imprisonment or death.

Chen is currently the only US citizen designated as "wrongfully detained".

"I have not been able to speak with my husband for over 600 days and am concerned for his health and well-being," Rong said in a statement through Global Reach.

In an interview with Reuters, she said Chinese authorities interrogated her husband more than 100 times on his work and that he wasn't allowed to see a lawyer for the first 13 months of his detention.

Born in China, Chen became a US citizens in 2011. He lives in Boston, Massachusetts.

He specialised in using seismological data to identify nuclear tests, and undertook several projects funded by the US government. Rong said he his work with colleagues in China was always done "transparently".

"He is doing precisely the kind of people-to-people engagement that the Chinese government says it wants," she added.

Chen's work included a study in December 2020 that looked at seismic data recorded across Asia, including China, to improve methods for nuclear-test monitoring and yield estimation.

According to Global Reach, there are "suspicions within the US government that Chen's arrest was spurred by China's conduct of nuclear tests in violation of the Comprehensive Nuclear Test Ban Treaty".

The group said Chen's expertise would give China "an opportunity to learn as much as possible about US seismic detection methodologies so they can establish countermeasures that allow them to circumvent the treaty".

The treaty seeks to ban all nuclear explosion tests on Earth, but several "nuclear-capable" states have not ratified it yet. Among them are the US and China, which have both established voluntary moratoriums against explosive nuclear testing.

In June 2020, during Donald Trump's first presidency, his administration accused Beijing of conducting a covert underground nuclear test at the Lop Nur facility in the country's north-west. China dismissed the claims as unfounded and politically motivated.

The Foley Foundation, another US-based hostage advocacy group, said Chen's health is a concern, noting that he suffers from diabetes, high blood pressure and high cholesterol.

"He needs reliable access to treatment and care that is not available while he is unjustly incarcerated," the group said.

US Senator Edward Markey, a Massachusetts Democrat, said Beijing's "treatment of Chen has undermined [its] partnership [with the US] and may deter other academics from engaging with their colleagues in China".

"It is my hope that increased attention on his unjust detention will force the Chinese government to do the right thing and release Chen," he wrote in a statement published on Tuesday.

Details around Chen's detention came a month after China confirmed that it had arrested another US scholar, Min Zin, director at a Myanmar-focused think tank.

Beijing has accused Min Zin of spying and endangering Chinese national security.