PHNOM PENH (Reuters) - Cambodia on Monday jailed an opposition senator for seven years over his Facebook posting of a fake government pledge to dissolve the border with Vietnam, a verdict opposition members say could fuel tension in the Southeast Asian nation.

For centuries, Cambodia has fretted about its much bigger neighbors, Vietnam to the east and Thailand to the northwest, encroaching on its territory. The issue remains emotive and many Cambodians are suspicious of both countries.

The Phnom Penh Municipal Court found Hong Sok Hour, 59, guilty of forging and publishing public documents, and of incitement to cause instability, when he posted a 1979 Cambodia-Vietnam treaty on social media site Facebook and said the two countries had agreed to dissolve their mutual border.

There was no immediate comment from Sok Hour, a member of the opposition Cambodia National Rescue Party (CNRP), who was not in court, or represented by lawyers, when the verdict was handed down.

The opposition in Cambodia has for years accused Prime Minister Hun Sen of ceding land to Vietnam in the hope of turning voters against him.

The prime minister, who rose to power in the 1980s as part of a government that was backed by Vietnam, has dismissed the accusations.

Sok Hour, who is also a French citizen, was detained over the posting in August. Last year, he was arrested following an order from the long-serving Hun Sen after he posted a video clip on the Facebook page of CNRP president Sam Rainsy.

Tension between Cambodia's two main political parties has risen in recent months, with the opposition complaining of a crackdown by the ruling party in a bid to intimidate critics before a general election in 2018.

Hun Sen and his government have repeatedly denied wrongdoing.

The court's decision would worsen Cambodia's political crisis, said Son Chhay, a senior CNRP member.

"There won't be any political solution any time soon," Chhay told reporters at the court following the verdict, adding that Sok Hour had admitted to mistakenly using the public documents but that he should not have been given a seven-year term.

International rights groups, including Human Rights Watch, had called for the charges against Sok Hour to be dropped.

(Reporting by Prak Chan Thul; Editing by Amy Sawitta Lefevre and Clarence Fernandez)