Cambodia?s main opposition leader is rejecting a Phnom Penh court?s decision to sentence him to 10 years in prison for altering documents and spreading disinformation involving a border dispute with Vietnam.

The court convicted Sam Rainsy of altering a map to show that Vietnam?s border is encroaching onto Cambodian land. The judge, declaring that the actions would have discredited the government, fined the opposition leader more than $1,000 and ordered him to pay at least $14,000 to cover the government's legal fees.  

In an interview with VOA Thursday, Sam Rainsy called the charges politically motivated.

"I am not surprised at all, because this is not the first time that I am sentenced to a prison term," Sam Rainsy said from Paris, where he lives in self-imposed exile. "This is actually the third or the fourth time that the court is being used as a political tool by the ruling party to crack down on the minority."

In January, the opposition leader was sentenced to two years in prison for uprooting posts demarcating the border with Vietnam. Sam Rainsy says he is being unlawfully prosecuted by Cambodian Prime Minister Hun Sen.

?I am doing my duty in defending Cambodian farmers who have been losing their rice fields along the border with Vietnam because of border encroachment,? Sam Rainsy said. The 61-year-old accused the government of covering up the loss of Cambodia land to Vietnam.

Vietnamese troops occupied Cambodia for 10 years after toppling the genocidal Khmer Rouge regime in 1979. Some Cambodians view Hanoi as a liberating force, while others consider it a self-serving government that is using Cambodia?s leader as a puppet. Cross-border relations are a political flashpoint that generally flare during election seasons.

The opposition says Mr. Hun Sen?s ruling party is taking action because it is concerned about the group?s influence in the 2013 general election.

Brad Adams, the Asia director for Human Rights Watch, also expressed concern Thursday about the treatment of Cambodia?s opposition.

?It's another example of the Cambodian government and Hun Sen, the prime minister, personally trying to persecute political opponents who have something different to say than the government does on various subjects," Adams told VOA.

Mr. Hun Sen is a former Khmer Rouge fighter who defected to Vietnam in the late 1970s. He returned to Cambodia to work in the Hanoi-backed government, becoming prime minister in 1985. Sam Rainsy is his chief rival, but has made few gains in his bid to challenge the prime minister.