BEIJING—Chinese authorities have detained an American citizen for six months over allegations of spying and theft of state secrets, her husband and lawyers said Monday in the first public disclosure on the case.

Phan Phan-Gillis—a 55-year-old Houston-based businesswoman of Chinese descent—was on a trade delegation from Houston traveling the southern coastal city of Zhuhai in March when she was taken into custody, according to Jeff Gillis and her Chinese lawyers. She was transferred to the inland city of Nanning where on Sunday authorities placed her under criminal detention, a procedure that often leads to a formal arrest, according to Mr. Gillis and the lawyers.

Mr. Gillis and the lawyers said they haven't been given details on what Ms. Phan-Gillis is alleged to have done. State-security officials have given few specifics about her case and haven't allowed her to directly contact her family, friends and lawyers, Mr. Gillis said.

She has been granted regular visits from U.S. consular officers. One of them, Tyler Allen, who has been handling the case declined to comment. The U.S. Embassy in Beijing and the State Department's Bureau of Consular Affairs didn't immediately respond to requests for comment.

Phone calls to China's Ministry of State Security weren't answered. A propaganda official at Nanning's public-security bureau said she didn't have knowledge of the case.

"My wife isn't a spy or a thief," Mr. Gillis said in an interview. "She is a hardworking businesswoman who spends huge amounts of time on nonprofit activities in Houston and China."

At the time of her detention, Ms. Phan-Gillis was traveling in China as part of a five-person trade delegation that included businessmen and Houston municipal officials, according to Mr. Gillis and Ed Gonzalez, a Houston city council member who led the delegation. Mr. Gonzalez, in an interview, said nothing unusual happened during the roughly weeklong trip—which included meetings with local officials in the cities of Qingdao and Shenzhen—until Ms. Phan-Gillis failed to emerge from immigration checks at Zhuhai.

Mr. Gillis, a 53-year-old oil-and-gas services manager in Houston, said he didn't publicize his wife's detention at first so as not to jeopardize efforts by officials to secure her release. He decided to go public ahead of Chinese President Xi Jinping's arrival in the U.S. on Tuesday for a weeklong visit that includes talks with President Barack Obama.

China's definition of state secrets is broad, and foreign nationals—including U.S. citizens—have in the past fallen foul of the law in their handling of information on the mainland. In April, a U.S. geologist jailed in China for more than seven years after being convicted of trading in Chinese state secrets was released and deported to the U.S. That case underscored Beijing's deep sensitivities about information it considers secret, and the limits of foreign diplomacy in influencing such cases.

Ms. Phan-Gillis—born in Vietnam to descendants of migrants from China's southern Guangdong province—moved to Houston in the late 1970s and became a U.S. citizen in the early 1980s, according to Mr. Gillis, her husband of 13 years. For decades, Ms. Phan-Gillis, who is known by the nickname Sandy, ran her own business consultancy and also had interests in project management and financing.

As president of the Houston Shenzhen Sister City Association—a group that promotes business ties between the two cities—she has helped arrange visits to China by business, cultural and sports delegations from Houston and vice versa, according to Mr. Gillis and public records published online by the Houston city council.

--Olivia Geng contributed to this article.

Write to Chun Han Wong at chunhan.wong@wsj.com

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(END) Dow Jones Newswires

September 21, 2015 13:05 ET (17:05 GMT)