Opposition lawmakers say they have seen evidence of toxic chemicals being used on rural land near Cambodia’s frontier with Vietnam, amid claims by local villagers the attacks were facilitated by Vietnamese authorities as part of an ongoing border dispute.

The visit came after a complaint was filed by 70 families in the Memot district of Tbong Khmum province on April 19, in which they called for assistance from MPs from the Cambodian National Rescue Party (CNRP).

“It is really regretful the Vietnamese armed forces have protected their people so they can enter the area and use chemicals to destroy more than 10 hectares of plants, cassava, rice and so on,” said Mao Monivann, the CNRP representative for Kampong Cham who lead the 200-person group on the visit.

Monivann said he would seek to summons the ministers of Foreign Affairs, Defence and Interior to the National Assembly so they could clarify the situation. He said he would also be asking for diplomatic communications to be sent to Vietnam on the matter.

The land has been cultivated by Cambodians since the mid-1980s, villagers say, but Vietnamese authorities consider it a disputed “white” zone, and since the 1990s there have been claims of obstructions to Cambodian activities, including the use of chemicals to destroy crops.

A Vietnamese spokesperson could not be reached for comment.