The United States says it is “deeply concerned” at the conviction of a prominent Cambodian land rights campaigner who was sentenced to 20 years in prison on insurrection charges.

In a statement Monday, the State Department characterized the sentence against Mam Sonando as “harsh” and called on the Cambodian government to immediately release the 71-year-old.

A Cambodian court on Monday convicted Sonando of inciting villagers to try to form their own state following a land dispute in eastern Cambodia earlier this year. Sonando denied the charges.

His radio station, known as Beehive Radio, sometimes broadcasts material critical of Prime Minister Hun Sen, who has ruled Cambodia since 1985.

The Cambodian Center for Human Rights slammed the conviction as being politically motivated. It said there is no evidence Sonando was involved in the unrest, which the government viewed as a rebellion.

Amnesty International called the conviction “shocking and baseless,” saying it reflects the deteriorating situation of freedom of expression in Cambodia.

Cambodia has carried out an intensifying wave of sometimes violent forced evictions affecting tens of thousands of people. Activists say officials are increasingly cracking down on those who challenge the land grabs.