China has sentenced an American geologist to prison for violating the country's broad, and vague, state secrets law.

Chinese authorities detained 44-year-old Xue Feng in November 2007 on charges of attempting to acquire a database of information about China's oil industry.

Xue is a naturalized U.S. citizen. He was working for an American energy and engineering consulting firm, IHS, at the time. The database was classified as a state secret after Xue had purchased it.

A Beijing court ruled him guilty more than a year ago, but sentencing has been repeatedly delayed.

On Monday, Xue was sentenced to eight years in prison. Three Chinese nationals, who were convicted with him, were also sentenced to prison; one to eight years and the others to two and half years.

US Embassy spokesman Richard Buangan says the American government is very concerned.

"The embassy is dismayed by Dr. Xue's sentence, and we remain concerned about his rights to due process under Chinese law," Buangan said.

Legal experts advising Xue's family say that after he was detained, state security agents tortured him, including burning him with lit cigarettes.

Since Xue's detention nearly three years ago, Buangan says U.S. officials visited him 30 times to check on his welfare. He says Ambassador Jon Hu ntsman has taken a strong interest in the case and was in the courtroom for Monday's sentencing.

"Ambassador Huntsman said earlier this morning that he visited Xue Feng several times during the past half year," said Buangan. "Dr. Xue has stayed strong during this difficult time and the ambassador spoke with him and his family, with whom he hopes to be reunited soon."

Chinese officials have wide authority to define what state secrets are, which makes it difficult for foreigners and Chinese alike to know when they are crossing the line. Secrets often include economic and business data considered public information in most countries. And information can be declared secret even if it has already been published.

The notoriously vague state secrets laws received international attention last year, when four employees of the Australian mining giant Rio Tinto were detained for stealing state secrets during tense iron ore negotiations.

They were later convicted of lesser charges of receiving kickbacks and stealing commercial secrets.

Among the defendants in the Rio Tinto case were three Chinese nationals and Stern Hu, an ethnic Chinese who had become a naturalized Australian citizen.