BEIJING • An American consultant who has been detained in China for more than a year has been formally charged with spying, news that could further complicate United States-China ties ahead of President Barack Obama's trip to Asia.

Sandy Phan-Gillis, 56, of Houston is a naturalised US citizen of Chinese ancestry. She was arrested in March last year while travelling in southern China with a trade delegation and had been held without charge till now.

"Based on our understanding, Phan-Gillis, because of suspected espionage, has been charged according to law by the relevant Chinese department," Ms Hua Chunying, a spokesman for China's Foreign Ministry, said in a news conference on Tuesday.

"China is a country ruled by law. The relevant Chinese department will handle the case strictly according to law," she added.

Ms Hua did not offer any further comment on the nature of the alleged crimes.

In a letter dictated to a US consular official and released by her husband, Mr Jeff Gillis, Phan-Gillis said the charges against her were political.

"I am accused of being a spy for the US government. I have never been a spy," she was quoted as saying.

Mr Gillis has said China claims she acted as a spy for a foreign government in 1996. He has called the charges against her "false" and expressed concern for her safety and well-being in detention.

News of the formal charges comes just days before Mr Obama is scheduled to land in China for Group of 20 meetings to be held in the southern city of Hangzhou.

He and China's President Xi Jinping are scheduled to hold bilateral talks on Saturday before the main meeting, according to the White House.

The Phan-Gillis case could complicate a relationship already strained by issues such as espionage, cyber security and US involvement in China's territorial disputes with other countries over the South China Sea .

In July, the US authorities sentenced a Chinese national, Su Bin, to 46 months in prison for conspiring to steal US military secrets from defence contractors. The Chinese Foreign Ministry said the case was driven by "ulterior motives".

Separately, Chinese Premier Li Keqiang said yesterday a Canadian citizen detained in China for two years on suspicion of spying would be treated humanely and his case would be handled in accordance with the law.

China indicted Canadian citizen Kevin Garratt in January on charges of spying and stealing state secrets. He was detained in August 2014 near China's sensitive border with reclusive North Korea.

Mr Li, who was at a joint news conference with Canadian Prime Minister Justin Trudeau in Beijing, did not mention Garratt by name but it is rare for a top Chinese leader to comment publicly on such a highly sensitive subject.

WASHINGTON POST, NEW YORK TIMES, REUTERS