BEIJING (AFP) - A US businesswoman has been held for six months in China over alleged espionage, her supporters said, revealing a case that could complicate Chinese President Xi Jinping's state visit to the United States, beginning on Tuesday.

China's ministry of state security detained Sandy Phan-Gillis in March and she is being investigated on accusations of "spying and stealing state secrets", according to the website savesandy.org, which provides information on her and her case.

Phan-Gillis was held while crossing the border to Macau at the end of a visit to China by a trade delegation from the Texas oil capital Houston, where she is a member of the mayor's International Trade and Development Council, the site said.

The five-member group had visited other Chinese cities including Beijing, Guangzhou and Shenzhen and only Phan-Gillis was detained, it added.

"Sandy is not a spy or a thief," her husband Jeff Gillis said on the website.

Foreign ministry spokesman Hong Lei confirmed the investigation at a regular briefing on Tuesday.

"The person you mentioned... is suspected of carrying out activities endangering national security, and is currently being investigated by relevant departments," he said.

"We hope that the outside world will respect China's handling of this case according to law."

It was not clear why the case had not been publicised until Monday, when the first announcements were posted to the site.

But the news came as Xi embarked on a visit to the United States beginning with a stop in Seattle before heading to Washington for talks with President Barack Obama.

"Sandy has now been detained for over six months, and she has not been allowed to see or speak with friends, family, or even her lawyers in that time," the website says.

She was formally arrested at the weekend, the website said, but added that no charges had been filed and earlier appeals for her release had received no response.

"Chinese authorities have conceded that they don't have sufficient evidence to file formal charges against Sandy, yet they have not released her," it added.

Phan-Gillis has family origins in southern China but was born in Vietnam, the website says, leaving the country in the late 1970s as part of the exodus of "boat people" who fled Communist rule.

Other foreign citizens have run afoul of China's powerful security officials.

Feng Xue, a Chinese-born US geologist who spent more than seven years in a Chinese prison after being convicted on state secrets charges, was released in April and deported.

Australian national Stern Hu, an executive with the mining giant Rio Tinto, was sentenced to 10 years in prison in 2010 on bribery and trade secrets charges.

A Canadian Christian couple who ran a coffee shop in the Chinese border city of Dandong, and had aided Christians fleeing North Korea, were detained on espionage charges last year.

China announced that the wife was released on bail in February though her husband had been placed under formal criminal detention.

Phan-Gillis has been visited once a month by officials from the US Consulate in Guangzhou, the website said. Hong, the foreign ministry spokesman, said that US officials had met with her six times.

The website describes Phan-Gillis as in "very poor health" and suffering from ailments including high blood pressure.

"She is currently well and healthy," Hong said, also stressing that her "rights are being respected".

The US embassy in Beijing referred inquiries to consular affairs officials in Washington.