SaveSandyorg Sandy Phan-Gillis of Houston, Texas has been charged with espionage by authorities in China.

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 tension.

Sandy Phan-Gillis, from Houston, Texas, who has 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 is unclear what violations the charge covers.

The government has chided the U.N. Working Group on Arbitrary Detention for saying her detention violated international human rights norms.

The U.S. State Department has urged China to resolve the case “expeditiously”.

The charge comes amid heightened tension in U.S.-China relations, dogged by issues from differences over territorial disputes in the South China Sea to the sentencing in the United States of a Chinese national for conspiracy to hack sensitive military information.

The Chinese man, Su Bin, 51, was jailed for 46 months in July after pleading guilty to conspiring to hack into the computer networks of major U.S. defense contractors.

U.S. President Barack Obama will arrive in China on Saturday for a G20 summit in the city of Hangzhou.

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. Her husband, Jeff Gillis, has said she is not a spy or a thief.

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.