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An American woman has been detained in China for months on suspicion of spying, officials said Tuesday.

The Chinese Foreign Ministry said that Sandy Phan-Gillis was "suspected of engaging in activities that have harmed China's national security," without giving any more details about the allegations.

She was "assured of all her rights…is in a good state of health and is cooperating with the investigation," the Foreign Ministry added in a statement.

The U.S. State Department said it was aware of Phan-Gillis' detention in southern China and had been providing "all possible consular assistance" to her since her arrest on March 20.

Gabrielle Price, spokesperson for the State Department's Bureau of East Asian and Pacific Affairs, said Phan-Gillis had been visited six times by consular officials. Price directed questions about the charges to local Chinese authorities.

Another State Department spokesman, Mark Toner, said the White House raised the issue with the Chinese foreign ministry "and have not received what we believe to be an adequate response about the charges against her."

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News of the detention came as China's President Xi Jinping was due to arrive in the U.S. for his first state visit.

Phan-Gillis' husband, Jeff Gillis, has appealed for his 55-year-old wife's release and stressed her innocence.

"My wife is not a spy, she is not a thief," he told NBC station KPRC on Monday. "I am concerned, though, that if we can't get her released this week, that it could very easily be years. And my wife is not guilty of anything — she needs to be let go."

Jeff Gillis said he has chosen to publicize his wife's detention to coincide with Xi’s visit in the hope that it will put pressure on Chinese and U.S. authorities, according to the Houston Chronicle.

An undated photograph of Sandy Phan-Gillis. savesandy.org

Phan-Gillis is from Houston, Texas. She was born in Vietnam but her family originates from China's Guangdong Province, according to a website set up by her family to fight the case. It describes her as a "businesswoman, and an entrepreneur."

She went to China in March with the trade delegation given her role as president of the Houston-Shenzhen Sister Cities Association and member of Houston mayor's International Trade and Development Council, according to a statement posted on the website.

The delegation — which included Houston's Mayor Pro-Tem Ed Gonzalez and his chief of staff — was traveling from Zhuhai to Macau on March 19 when Phan-Gillis was detained at a checkpoint. She was the only member of the group held, the statement said.

An undated photograph of Sandy Phan-Gillis. savesandy.org

Phan-Gillis spent six months in residential surveillance — similar to house arrest — in the southern city of Nanning before being formally arrested on the weekend of September 19-20 and placed in formal detention, according to the website.

The family statement claimed that no formal charges have yet been brought against Phan-Gillis and that the Houston local has been in poor health — even running out of her medication at one point before being resupplied.

The Phan-Gillis has been visited by an official with the U.S. Consulate in Guangzhou, according to the family statement.

Jeff Gillis told KPRC he has not spoken to his wife since March 23 — when she called to say she was extending her trip. She did not mention her detention and he said that he believes she was coerced.

"I am picturing them with the phone up to her head saying, 'This is what you can say,'" he told the NBC affiliate.