A human rights group Wednesday demanded the release of a US businesswoman held by China for alleged espionage since last year, after a UN committee said she had been "arbitrarily detained".

Sandy Phan-Gillis was seized in March 2015 while crossing the border to Macau at the end of a visit to China by a trade delegation from the Texas oil capital Houston, supporters said.

The American has been investigated on accusations of "spying and stealing state secrets", according to the website savesandy.org, which provides information on her case.

The UN Working Group on Arbitrary Detention (WGAD) said Sunday that "international norms relating to the right to a fair trial and to liberty and security" had not been observed in her case.

Violations by Chinese authorities were of "such gravity as to give the deprivation of liberty of Ms. Phan-Gillis an arbitrary character," it said in a report posted online.

Phan-Gillis was first held for six months in a secret location and later at a detention centre in the southern region of Guangxi, where she was initially placed in solitary confinement, WGAD said.

The report marked the first time in WGAD's 25-year history that it had "ruled that (Chinese) agents have arbitrarily detained an American citizen in violation of international human rights law," US-based rights group the Dui Hua Foundation said Wednesday.

Phan-Gillis has been allowed to see a lawyer for the first time, 14 months after she had been taken into custody, the group added in a statement.

The case has "badly damaged US-China relations," the group's executive director John Kamm said.

He added: "Dui Hua joins the WGAD, the American government, and members of Congress in calling for Sandy Phan-Gillis' immediate release".

Though Phan-Gillis, who was a member of Houston's International Trade and Development Council, is currently the only American being held on suspicion of spying in China, other foreign citizens have been accused of espionage.

Feng Xue, a Chinese-born US geologist who spent more than seven years in a Chinese prison after being convicted on state secrets charges, was released last year and deported.

Australian national Stern Hu, an executive with the mining giant Rio Tinto, was sentenced to 10 years in prison in 2010 on bribery and trade secrets charges.

A Canadian Christian couple who ran a coffee shop in the Chinese border city of Dandong, and had aided Christians fleeing North Korea, were detained on espionage charges in 2014. The husband was formally charged with stealing state secrets and his wife bailed.

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