PHNOM PENH(AFP) - Cambodian opposition leader Sam Rainsy was sentenced to five years in prison on Tuesday (Dec 27) over a post on his Facebook page, a conviction that makes his return from self-imposed exile unlikely.

The conviction caps a major crackdown this year on critics of the country's strongman leader Prime Minister Hun Sen, who has run Cambodia for more than three decades, often with the help of pliant courts.

Activists say Mr Hun Sen is trying to keep his opponents under pressure ahead of regional elections next year and a general election in 2018.

Scores of opposition politicians and rights group workers have been jailed or seen legal cases filed against them in recent months.

Rainsy, who currently lives in France, was convicted in absentia by Phnom Penh's Municipal Court on Tuesday for a post placed on his Facebook page by a third party about Cambodia's border with Vietnam, a highly contentious issue in the country.

Anti-Vietnamese sentiment runs high in Cambodia and parts of the border are still disputed. Mr Hun Sen's critics accuse him of being too close to Vietnam, a charge he denies and bristles at.

The case concerned copies of a 37-year-old treaty between Cambodia and Vietnam over their shared border that were posted on social media last year.

The Cambodians authorities said the treaty was a forgery and began arresting people who posted it, including opposition senator Hong Sok Hour who was jailed in November for seven years.

Hong Sok Hour posted the treaty on Rainsy's Facebook page, leading to charges being filed against the opposition leader.

Rainsy was convicted of being an accomplice in "falsifying public documents, using fake public documents (and) incitement causing unrest to national security", Judge Leang Samnath said during a hearing.

Two members of his Facebook team, who have fled the country, were also convicted in absentia.

Rainsy has been living in self-imposed exile in Paris since November 2015 after a court ordered him to serve jail time over a defamation conviction he says was politically motivated.

Cambodia has been plunged into a political crisis for much of this year. After Rainsy's flight, his deputy Kem Sokha was hit with a court case over a sex scandal.

He refused to appear in court and holed himself up in his party's headquarters for more than half a year until he was pardoned by the King earlier this month.

Mr Hun Sen, 64, has vowed to remain in power into his seventies.

But many Cambodians, particularly younger voters, have grown tired of endemic corruption and crony capitalism.

Rainsy's opposition party made huge gains in the 2013 elections and say they only lost because the vote was stolen - something Mr Hun Sen denies.