Image copyright SaveSandy.org Image caption A China-US business consultant, Sandy Phan-Gillis had been to China many times

An American woman has been held by authorities in China for the past six months accused of stealing state secrets, her husband has revealed.

Sandy Phan-Gillis's husband Jeff Gillis said he decided to go public about her circumstances ahead of Chinese President Xi Jinping's visit to the US.

China's Foreign Ministry spokesman Hong Lei confirmed the investigation in a regular briefing on Tuesday.

The US government has not commented. Her family deny the allegations.

The New York Times reports that she has now been formally arrested, citing her husband and lawyer.

House arrest

Mrs Phan-Gillis, a naturalised American born in Vietnam, was detained in March, during a trade visit to promote the Texan city of Houston.

According to Save Sandy, a website campaigning for her release, she has spent six months in so-called "residential detention", which it equates to house arrest.

Her husband says she is now being moved to a more secure facility.

Hong Lei said on Tuesday that Mrs Phan-Gillis was "suspected of carrying out activities endangering national security, and is currently being investigated by relevant departments".

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

Trade visit

Mrs Phan-Gillis was president of the Houston Shenzhen Sister City Association and consulted for Chinese and US businesses.

At the time of her detention, she was travelling with Ed Gonzalez, who led the trade delegation on behalf of Houston's mayor, as well as Chief of Staff Jerry Peruchini, Houston businessman Vincent Chau and Chinese businessman Gary Ge.

Mr Gonzalez told the BBC "there was nothing out of the ordinary" about the trip but described the sequence of events as they left China as "bizarre".

Image copyright SaveSandy.org Image caption The delegation travelled across China, including Beijing and Shenzhen, a sister city of Houston

They had been travelling around China for about a week when the group went through immigration control at the border between Guangdong and Macau, in Zhuhai, on 19 March.

The group became separated, regrouping on the other side - without Mrs Phan-Gillis.

After eventually moving on without her, a message was sent from her phone saying "something to the effect of 'I'm going to stay behind, to take care of personal matters'," Mr Gonzalez said.

He added that "it was strange, but she's been there [China] hundreds of times," so he thought little of it.

After returning to Houston several days later, Mr Gillis told them that the US consulate had informed him his wife had been detained, accused of stealing state secrets.

Her family vigorously deny the accusations, which they say have never been properly explained.

Image copyright SaveSandy.org Image caption Her family say she is being investigated for allegedly spying and stealing state secrets, but insist "Sandy is not a spy or a thief"

Consular visits

Since then, consular officials have been able to visit Mrs Phan-Gillis periodically, to check up on her health, which her husband says is poor, and to pass on messages.

The US embassy in Beijing would not comment on the story and referred the BBC to the US Department of State in Washington, which was closed at the time of enquiry.

She is not the only Westerner to have been accused of secrets offences in China in recent years.

Chinese-American geologist Feng Xue was released earlier this year after spending more than seven years in prison in China.

In 2010, Australian national and Rio Tinto mining company executive Stern Hu, also of Chinese origin, was sentenced to 10 years in prison on trade secrets and bribery charges.

A Canadian couple in the Chinese border town of Dandong, who helped Christians escaping North Korea, were accused of spying last year. The authorities released the wife on bail in February, but the husband was formally detained on criminal charges.