The US Embassy in Astana has criticized the Kazakhstan government over the treatment of a leading Kazakh opposition figure, the embassy announced on Tuesday.

Kazakh opposition leader Vladimir Kozlov was charged with “inciting social hatred” and sentenced on Monday to seven and a half years behind bars for his role in the Zhanaozen oil-workers’ protest last year.

The case was a “clear use of the criminal justice system to silence the opposition,” the embassy said in its statement, while noting the United States “values the strategic partnership” with Kazakhstan, and notes the country’s commitment to “increasing openness and democracy.”

Two other opposition leaders, Akzhanat Aminov and Serik Sapargali each received three-year suspended sentences. In total 37 people have gone on trial following the Zhanaozen protests, charged with crimes ranging from arson and theft to using violence against members of the public and officials.

At least 15 people are thought to have been killed and 100 injured in the Zhanaozen incident, after Kazakh police dispersed an oil workers’ protest in the town in December 2011.