• China is known for enforcing strict state secrecy laws.

China is known for enforcing strict state secrecy laws. (Photo : Getty Images)

An American woman detained in China since the past year has been charged with spying, China's Foreign Ministry announced on Tuesday, in the latest development in a case that as added to Sino-U.S. tensions.

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

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"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 said during a media briefing.

"China is a country ruled by law. The relevant Chinese department will handle the case strictly according to law," she added without further elaboration.

It remains unclear what violations the charge covers.

Beijing has also rebuked claims of the U.N. Working Group on Arbitrary Detention that Phan-Gillis's detention violates international human rights norms.

The U.S. State Department has urged Chinese authorities to resolve the case "expeditiously."

The charge comes amid increasingly strained relations between U.S. and China, dogged by issues such as 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.

51-year-old Sun Bin has been in detention 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 is scheduled to arrive in China on Saturday to attend the G20 summit in the city of Hangzhou.

In a letter transcribed by a U.S. consular official in China, Phan-Gillis said her detention was motivated by politics and not for any crime.

She visited China on a trade delegation from Houston and was arrested while attempting to cross from the southern city of Zhuhai to Macau. Her husband, Jeff Gillis, has said she is neither a spy nor a thief, according to the Huffington Post.

China's state secrecy laws are extremely broad, covering everything from industrial data to the birthdays of top Chinese leaders. Information can also be declared a state secret retroactively.

The country lacks independent oversight of its courts and law enforcement agencies, which answer to the ruling Communist Party.