Prime Minister Hun Sen hailed the agreements signed in Hanoi on Saturday as further steps towards ending conflicts and ensuring peace and development along the Cambodian-Vietnamese border for generations to come.

Hun Sen led a Cambodian delegation on a state visit to Vietnam from Friday to Saturday at the invitation of Vietnamese Prime Minister Nguyen Xuan Phuc.

He and his Vietnamese counterpart signed the Supplementary Treaty to the 1985 Treaty on the Delimitation of the National Boundary and the 2005 Supplementary Treaty, a joint statement from the two countries said.

The Protocol on the Demarcation and Marker Planting of the Land Boundary Between the Kingdom of Cambodia and the Socialist Republic of Vietnam was also inked.

However, the Supreme Court-dissolved Cambodia National Rescue Party said the 1985 treaty was illegal and ceded Cambodian territory to its eastern neighbour.

Hun Sen said the signing of the documents represented a “step forward”.

“The settlement of border issues between the countries moves a step forward with pride and fraternity. The achievements of border marker planting and boundary delineation between the two countries contribute to ending conflicts and disputes between the people of the two nations.

“I would like to declare solemnly that the resolving of areas where the border is not yet agreed on continues.

“The government of Cambodia stands firm on building Cambodia-Vietnam ties on the grounds of national and international law, with international implementation in recognising the principle of the unchangeable border left by the French colonial administration as a strong basis in solving border matters permanently.

“If we don’t make efforts to end the border issues, future generations will still face the chance of conflicts with our two countries having an unresolved border.

“It could even become the subject of political competition . . . peace and development along the border might not happen,” Hun Sen said at the signing ceremony of the supplementary treaty in Hanoi on Saturday.

“Prime Minister Hun Sen and Prime Minister Nguyen Xuan Phuc co-chaired a conference reviewing the progress made in Cambodia-Vietnam border demarcation since 2006 and the signing ceremony for the two legal documents on border matters,” the joint statement said.

The two parties also agreed to foster further cooperation on border management based on existing legal frameworks and mechanisms to build a border of peace, stability, friendship, cooperation and sustainable development, it added.

The two sides attached special significance to the signing of the documents, hailing the progress of border demarcation and marker planting – around 84 per cent complete – as a historical milestone.

“The two sides reaffirmed their commitment to the full observation of the border delimitation treaties and relevant border agreements signed between the two countries,” the statement said.

They also agreed to complete demarcation and marker planting for the remaining areas in the near future, it added.

Kin Phea, the director of the International Relations Institute at the Royal Academy of Cambodia, said on Sunday that Hun Sun’s visit had helped strengthen ties with Vietnam.

“Relations and cooperation between the two countries are better, and the visit also fostered increased trade and investment. The visit showed two-way support and we expect to see positive results from it,” Phea said.

The CNRP issued a statement on Saturday claiming the original 1985 Treaty on Delimitation of National Boundaries was illegal.

“[The CNRP, when in power] will review the illegal border treaty and the border markers that have been planted in the past with Vietnam. It will ensure Cambodian territorial integrity in line with the Paris Peace Agreements and the Cambodian Constitution,” the CNRP said.

Koy Pisey, the deputy head of the Border Affairs Committee, said the former opposition party could only attack the achievements of the government without good reason. She claimed the Cambodian people welcomed the signing of the treaty demarcating the border.

“The attack [by the CNRP] is nothing new because it did not recognise the border demarcation efforts from the beginning. It is an opposition group. It objects to everything whether big or small.

“But this is a historic matter for Cambodians. They are happy and applaud the results achieved by the government. We work clearly, seriously and properly,” Pisey said.

Cambodia and Vietnam, she said, share 270km of the border.

The Cambodian-Vietnamese joint statement said grant agreements had been signed on the construction of a new administrative building for the Cambodian National Assembly and parts of a drug treatment centre in Preah Sihanouk province.

Hanoi expressed appreciation to Cambodia for the continued support and non-discriminatory treatment given to people of Vietnamese origin living in the Kingdom.