BEIJING (Reuters) - An American businesswoman held in China since March last year has been charged with spying, China’s Foreign Ministry said on Tuesday, the latest development in a case that has added to U.S.-China tensions.

Sandy Phan-Gillis, from Houston, Texas, who is of Chinese ancestry and is a naturalized U.S. citizen, was arrested in March 2015 and had been held without charges since then.

“Based on our understanding, Phan-Gillis, because of her suspected crimes of espionage, has been charged according to law by the relevant Chinese department,” Chinese Foreign Ministry spokeswoman Hua Chunying told reporters at a regular briefing.

“China is a country ruled by law. The relevant Chinese department will handle the case strictly according to law,” she said, without elaborating.

It was unclear what violations the charge covers.

News of the charges against Phan-Gillis comes just ahead of a visit to China by U.S. President Barack Obama, who will arrive on Saturday for a G20 summit in the city of Hangzhou. Obama is scheduled to hold bilateral meetings with Chinese President Xi Jinping on Saturday.

Obama’s visit comes at a time of heightened U.S. tensions with China, particularly over Beijing’s extensive territorial claims in the South China Sea, but also over issues such as cyber spying.

In July, a Chinese man, Su Bin, 51, was sent to prison for 46 months in the United States after pleading guilty to conspiring to hack into the computer networks of major U.S. defense contractors.

A U.S. State Department official said the United States was “deeply concerned” about Phan-Gillis’ welfare and had repeatedly pressed China to provide further details of the case and to allow U.S. consular officers “full and unfettered” access to her.

“We urge Chinese authorities to explain the reasons for Ms. Phan-Gillis’ ongoing detention,” the official said.

The official said the United States was also calling on China to “to review and consider seriously” the recommendation of the U.N. Working Group on Arbitrary Detention that Phan-Gillis be released.

The Chinese government has chided the U.N. group for saying her detention violated international human rights norms.

The U.S. consulate in Guangzhou had been providing consular assistance to Phan-Gillis, including monthly consular visits, the State Department official said.

Her husband, Jeff Gillis, said the “charges are absolutely false,” and called for her release.

He said the charges include an accusation that his wife went on a spy mission to China in 1996. Her passport at that time shows that she made no trip to China that year, he said in a statement.

Phan-Gillis had said in a letter transcribed by a U.S. consular official in China that her detention was because of politics and not for any crime.

She visited China on a trade delegation from Houston and was detained while attempting to cross from the southern city of Zhuhai to Macau.

China’s state secret law is extremely broad, encompassing everything from industrial data to top leaders’ birthdays. Information can also be declared a state secret retroactively.

There is no independent oversight of China’s law enforcement authorities or courts, which answer to the ruling Communist Party.