Xue Feng, who had been serving an eight-year sentence in Beijing No. 2 Prison, was immediately deported upon leaving jail, the US-based Dui Hua Foundation said in a release dated Friday.

The rights group, which advocates clemency and better treatment for prisoners in China and had repeatedly raised Xue's case with Chinese officials, said that he arrived in the US city of Houston on Friday evening.

"Dui Hua is delighted that Dr. Xue has finally been reunited with his family in America after a terrible ordeal," the San Francisco-based group's executive director John Kamm said in the release.

The US embassy in Beijing said in an e-mail it was "aware of reports" that Xue had been released, but could add no more due to privacy considerations, directing media instead to Chinese authorities.

China's state-run Xinhua news agency did not immediately carry any articles on the release. A phone number for the Beijing No. 2 Prison could not be found and the institution's website had no information on Xue's release.

Xue, a Chinese-born US citizen, was first detained in November 2007 over the sale of a database on China's oil industry while working for US energy and engineering consulting firm IHS.

"We are pleased to hear that Mr. Xue has been released and allowed to return to his family in the United States," a spokesman for IHS told AFP in an email.

Both Xue and IHS had said in the past that they believed the database to be a commercially available product. It was only classified as a state secret after Xue had bought it, according to Dui Hua.

In February 2011, a Beijing court upheld his 2010 conviction and eight-year sentence.

In November 2012, Xue received a 10-month reduction in his sentence for good behaviour, Dui Hua said.

His release means that there are now no US citizens serving sentences in China on convictions of endangering state security, Dui Hua said.

The US repeatedly raised concerns over whether Xue's rights were being protected and whether he had access to a fair trial.

Rights activists say China routinely abuses its state secrets laws, often as a means of silencing government critics.

Xue's arrest and other cases have cast a spotlight on the dangers of doing business in China, especially for those born in China who take on a foreign nationality.

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, in a case severely criticised by Canberra.