PHNOM PENH, March 7 (Xinhua) -- Cambodia will take legal action against self-exiled former opposition leader Sam Rainsy for allegedly promising to cede four Cambodian provinces to Vietnam's Montagnard indigenous group if his then-Cambodia National Rescue Party (CNRP) won the July 2013 election, a spokesman said on Wednesday.

"This is a dreadfully treasonous act that must not be tolerable," Interior Ministry spokesman Gen. Khieu Sopheak told Xinhua, adding that the act was completely against the country's constitution.

"We are preparing to file a complaint to the court against Sam Rainsy," he said.

The spokesman said for such a treasonous act, if convicted, the former opposition leader could be jailed for life.

The spokesman's comments came a day after a video clip, along with an agreement on Montagnard Indigenous Rights signed on April 14, 2013, in the United States between Sam Rainsy and Kok Ksor, head of the U.S.-based Montagnard Foundation, was posted on a Facebook.

In the video clip at the signing ceremony, Sam Rainsy said the Montagnards lived in Cambodian provinces of Mondulkiri, Ratanakiri, Kratie and Stung Treng, and he thanked them for supporting the then-CNRP.

"When we establish a new government, we will ensure that the Montagnards - the Degar people - will live as free citizens of Cambodia. Their forests, their mountains will be returned to them as the masters of the land like (the) Cambodian people," he said in the video clip.

Montagnard people, also known as Degar people, are mostly Christian ethnic minority groups that live in Vietnam's mountainous Central Highlands region.

Sam Rainsy, 68, has been living in self-imposed exile in France since November 2015 to avoid at least eight-year prison sentence for defamation and incitement cases.

On Nov. 16 last year, the CNRP was dissolved by the Supreme Court, more than two months after Kem Sokha, Sam Rainsy's successor, was arrested and charged with "treason" for conspiring with a foreign power in an attempt to overthrow the government.