Chinese foreign ministry officials say businessman Xie Chunren, a resident of the U.S. state of New Jersey, was detained back in May in Sichuan province on suspicion that he had engaged in what officials call "activities harmful to the security" of China.
The authorities, however, have yet to charge Mr. Xie, a Chinese-born American citizen.
A foreign ministry spokeswoman, who declined to give her name, says Xie Chunren was placed under house arrest on May 31.
She says he will be dealt with according to Chinese law.
Jerome Cohen is an expert on Chinese law at New York University, and is in contact with Mr. Xie's family. In an earlier interview, Mr. Cohen said China's laws on matters of national security make it possible for authorities to hold suspects without filing charges for extended periods of time.
"The problem is, who says what is a state secret, especially [at] a preliminary stage of the investigation? If the Chinese have it arranged, it is only the police," said Jerome Cohen. "Nobody can challenge that determination."
A U.S. embassy spokeswoman says authorities have allowed U.S. consular officials to visit Mr. Xie three times since he was detained. She said U.S. diplomats are working to ensure that his rights are respected.
China often accuses Chinese-born foreigners of spying for rival Taiwan, which Beijing claims as a part of Chinese territory, as a way of limiting their activities in China. Among those arrested recently is Ching Cheong, a Hong Kong journalist who mainland authorities say has confessed to spying on behalf of Taipei.