Cambodia’s Ministry of Foreign Affairs has demanded Vietnam stop encroaching into disputed border regions, citing five “illegal” irrigation ponds in Ratanakkiri province “dug deep inside Cambodian territory”.

In a letter to the Vietnamese Embassy in Phnom Penh, the ministry urged Vietnam to cease working in Pak Nhay Commune, O Yadav district, an area recently highlighted by opposition lawmakers as an example of Vietnamese encroachment on the border.

“Pending the demarcation of the border, the areas which have not been demarcated should not be changed,” the letter said, noting the same demand was made by Prime Minister Hun Sen during his meeting with Vietnam politburo member Le Hong Anh on June 9.

According to the letter, a report by the Interior Ministry, Cambodian Joint Border Commission and Ratanakkiri authorities found the ponds “dug by Vietnamese authorities” were between 380 metres and 545 metres inside the Kingdom.

Cambodia chairman of the joint-border committee Var Kimhong told local authorities to stop any Vietnamese working in the area and report any attempts to the committee for “urgent action”.

“We plan to send border expert officials to measure the border at that area on June 24,” Kimhong said, urging Vietnam “not to move markers”.

Last month government officials accused the CNRP – which has often called for the controversial 2005 border treaty to be scrapped – of fomenting anger and damaging bilateral relations over the Vietnam border.

However, the government appears to be taking a tougher stance, which independent analyst Ou Virak said could be a strategy to deflate the appeal of the CNRP’s populism.

CNRP lawmaker Ou Chanrith, who today will lead 100 people to a disputed cassava plantation in Svay Rieng province said the letter was a “good start but they need to do more, there are other areas threatened”.

The Vietnamese Embassy was unreachable for a comment.