WASHINGTON, May 8 (AP) -- The U.S. government rebuked China on Thursday over the sentencing of a senior Tibetan Buddhist monk to six years in jail, saying that his detention was illegal even under China's own criminal code.

"The United States is deeply disturbed by this decision," said State Department spokesman Nicholas Burns.

The conviction of Chadrel Rinpoche, along with two aides, for alleged separatism and leaking state secrets was seen as a warning to Tibetan clerics, many of whom remain deeply loyal to the exiled Dalai Lama.

The Chinese Communist Party is in the midst of a three-year campaign to suppress pro-independence sentiment in Tibet and discredit the Dalai Lama, according to U.S. officials.

"We would note that Mr. Rinpoche has been detained for nearly two years apparently, we think, in violation of Chinese law," Burns said.

He noted that under the provisions of China's revised criminal law, which went into effect Jan. 1, such a lengthy detention period would have required approval by a committee of the National People's Congress, upon a request by the state prosecutor.

"That didn't happen," Burns said.