President Obama shakes hands with Chinese President Xi Jinping at the conclusion of their joint news conference in Beijing in November 2014. (Pablo Martinez Monsivais/AP)

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 U.S.-China ties ahead of President Obama’s trip to Asia.

Sandy Phan-Gillis, 56, of Houston was arrested in March 2015 while traveling in southern China with a trade delegation and has been held without charge since.

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

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

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

In a letter dictated to a U.S. consular official and released by her husband, Jeff Gillis, Phan-Gillis said the charges against her were political. “I am accused of being a spy for the U.S. government,” she said. “I have never been a spy.”

Jeff Gillis has said that 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.

In July, a U.N. committee on arbitrary detention said China violated Phan-Gillis’s rights by holding her without cause and limiting her access to lawyers.

Responding to a question about the committee’s findings, State Department spokesman John Kirby called for Beijing to resolve the case quickly.

“We’re certainly concerned about her welfare and her lengthy detention without trial, and we urge China to resolve this case expeditiously and to ensure that Ms. Phan-Gillis continues to have full access to an attorney,” he said July 6. “Senior U.S. government officials have raised her case with senior Chinese government officials on multiple occasions, and I can assure you we’ll continue to do so.”

News of the formal charges comes just days before Obama is scheduled to land in China for Group of 20 meetings to be held in the southern city of Hangzhou. The case could complicate a relationship already strained by issues such as espionage, cybersecurity and the South China Sea. 

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

Gu Jinglu contributed to this report.