WASHINGTON (Reuters) - A Houston, Texas, businesswoman held in China since March last year is expected to be charged with violating its national security, Newsweek reported on Thursday, citing a U.S. State Department official.

Sandy Phan-Gillis, who has Chinese ancestry and is a naturalized U.S. citizen, was arrested in March 2015 while about to leave China for Macau and has been held without charges since then.

“Chinese authorities have informed us that they will bring a case against Ms. Phan-Gillis to the Nanning Intermediate People’s Court,” Newsweek quoted the State Department official as saying. It said the official had no information on the charges or date.

“We urge China to resolve this case expeditiously and to ensure that Ms. Phan-Gillis continues to have full access to an attorney,” said the official, who spoke on condition of anonymity because of the case’s sensitivity, Newsweek said.

A Washington spokeswoman for the State Department said it was looking into the report.

If confirmed, the charges would come amid heightened U.S.-China tension following a ruling on Tuesday by an international court in The Hague that China had no historic claim to large areas of the South China Sea and had violated the Philippines' economic and sovereign rights. Beijing has rejected the finding.

China also said last week said U.N. authorities should respect its judicial independence after a U.N. agency said Phan-Gillis had been detained arbitrarily.

In a statement that cited an unidentified source, the U.N. Working Group on Arbitrary Detention said Phan-Gillis had not been allowed to speak to a lawyer or family members regularly and had recently been hospitalized after suffering a heart attack.

China's state secrets 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.

(Reporting by Eric Walsh and David Brunnstrom; Editing by Leslie Adler)