In an unusual reversal of rhetoric, Prime Minister Hun Sen publicly questioned the loyalty of long-time ally Vietnam on Wednesday, while ordering the interior and justice ministries to thoroughly investigate former opposition leader Sam Rainsy for treason for allegedly colluding with the Kingdom’s eastern neighbour.

Rainsy was accused of treason by government officials last week over a 2013 video in which he pledged to award a degree of autonomy to Montagnard minorities living in Cambodia’s northeastern provinces.

“You accuse Hun Sen of cutting land to Vietnam, but now we found your true face,” the premier said in a speech opening a new bridge in Kampong Cham.

Rainsy, who has lived abroad since 2015 to avoid a slew of politically tinged convictions, denies offering to cede land, and claims that he simply pledged to allow ethnic minorities to follow traditional ways of life after years of losing land and forests under Hun Sen’s administration.

The promise of “autonomy” was made at an event with a pro-Montagnard activist in the US, and involved a written pledge that largely quoted the UN Declaration on the Rights of Indigenous People.

Following the resurfacing of the video, Facebook users and pro-government media outlets this week went on to claim Rainsy held illicit meetings with officials from Vietnam’s Ministry of Foreign Affairs in 2003 and 2004.

“Why do you go to do a secret negotiation? At the same time you insult me as a Vietnamese puppet?” Hun Sen asked.

He said Rainsy, the former president of the now-dissolved CNRP, pledged citizenship to ethnic Vietnamese living in Cambodia, a position with which he professed to disagree.

“I will question our friend Vietnam, whether they are actually loyal to me and Cambodia,” he said.

The spat marks a major role reversal for Hun Sen and his long-time political nemesis. Hun Sen’s government was originally installed by the Vietnamese following Vietnam’s overthrow of the murderous Khmer Rouge regime, from which the premier had defected. The prime minister has enjoyed a cosy relationship with his neighbour for decades, while Rainsy for years has accused the government of ceding territory to Vietnam and has repeatedly used anti-Vietnamese rhetoric to whip up his base – sometimes to the point of drawing accusations of racism.

Cambodian political analyst Meas Nee said that the conflict might be part of a broader regional shift, and an attempt to discredit Rainsy in the eyes of his supporters.

“In the past Sam Rainsy blamed Hun Sen for allocating land to Vietnam, and now Hun Sen blames Sam Rainsy,” Nee said, adding both men were trying to “undermine” the other.

Nee said the premier’s strong words against Vietnam, meanwhile, could be indicative of a diplomatic schism developing between the two countries.

“When you look at it from a distance, one side is moving towards the US and another is moving towards China,” he said, referring to Vietnam and Cambodia, respectively.

This might “affect the trust between Hun Sen and Vietnam”, he added.

San Chey, country director of the Affiliated Network for Social Accountability, agreed that Hun Sen appeared to be attempting to shift lingering animosity over his relationship with Vietnam onto Rainsy.

“I think that for this case, [Hun Sen] likely aims to frame Sam Rainsy, because Cambodia usually considers Vietnam as a good friend after the 7th of January, 1979,” Chey said, referring to the date of the Khmer Rouge’s collapse.

With Rainsy’s former party forcibly dissolved, and its more than 3 million supporters abruptly disenfranchised ahead of this year’s national election, Hun Sen needs to fully discredit his now-scattered opponents in order to lend his anticipated electoral victory a semblance of legitimacy, said Dr Paul Chambers, a regional expert at Thailand’s Naresuan University.

Chambers theorised that Hun Sen had known of Rainsy’s pledge for years, waiting for the proper time to use it.

“Hun Sen suddenly realizes that simply fixing the election isn’t enough – he must destroy the image of the opposition and paint them as treasonous. That is what he did to Kem Sokha; this is what he now does to Sam Rainsy,” he said in an email.

In an email on Wednesday, Rainsy dismissed Hun Sen’s treason allegations as “inventive statements” meant to “divert public attention” from the ongoing political crackdown.

He acknowledged that he met with Vietnamese officials at that time but did not elaborate on the details. Instead, Rainsy said that the meetings prove he never harboured any “anti-Vietnamese sentiment as some ignorant or ill-intended people allege”.

Khieu Sopheak, spokesman for the Ministry of Interior, said legal experts are preparing a complaint against Rainsy “soon”, but would not give an exact date.

Updated: 6:42am, Thursday 15 March 2018