Thai security agencies see rights strategy in Cambodia’s UNCLOS move

Thai security agencies have analysed two reasons behind Cambodia’s decision to ratify the 1982 United Nations Convention on the Law of the Sea, or UNCLOS, on Friday (March 6, 2026), despite having been a State Party since 1983, saying the move was intended to preserve and assert rights.

The first factor was Thailand’s announcement cancelling the memorandum of understanding between the Thai state and the Cambodian government on the area of overlapping claims to the continental shelf, or MOU44 (the 2001 MOU), and returning to UNCLOS.

The second was Vietnam’s plan to reclaim land from the sea to build a road and bridge linking the mainland with Phu Quoc island over a distance of more than 40 kilometres, as part of infrastructure development because the area has been designated as the venue for the 2027 Asia-Pacific Economic Cooperation (APEC) meeting, to be attended by leaders from 21 economies including the United States, China and Russia.

As for Phu Quoc, Cambodia has previously claimed rights over its waters because the island is located in the Gulf of Thailand. Geographically, it is about 24 miles (38 kilometres) from the Cambodian mainland. Still, Vietnam has been able to maintain possession through military and administrative advantages, even though it is 35 miles (about 56.5 kilometres) from the Vietnamese mainland.

Vietnam’s retaliatory strategy is aimed at countering Cambodia over the excavation of the Funan Techo Canal, which is planned to open in 2028 despite objections, as it would cause currents at the Mekong River mouth, or the Nine Dragons River, to change direction.

Documents the Cambodian government submitted to the Mekong River Commission (MRC), an intergovernmental organisation that coordinates, manages and develops water resources in the Lower Mekong Basin and whose members are Thailand, Lao PDR, Vietnam and Cambodia, show that the Funan Techo Canal project covers 180 kilometres and will link waterborne transport routes.

It starts from the Mekong River at Prek Takeo, about 30 kilometres south-east of central Phnom Penh, passes via Phnom Penh port on the Bassac River and through Kandal, Takeo and Kampot provinces to Kep, a southern coastal city of Cambodia on the Gulf of Thailand. The project also directly links Phnom Penh with the port in Sihanoukville, the country’s only deep-water port, as well as the new port in Kampot province.

The Funan Techo Canal has a waterway 100 metres wide and 5.4 metres deep, with a two-lane channel for cargo vessels at a depth of 4.7 metres. It also includes related components such as three watergates, 11 bridges and pedestrian walkways totalling 208 kilometres.

This is not unlike the case of Cambodia’s construction of a sediment-trapping dam, which affected Thai sovereignty and changed the ecosystem by blocking sediment from flowing down to Ban Hat Lek in Khlong Yai district of Thailand’s Trat province. It caused about 3,000 square metres of land to disappear, while a resort and casino in Koh Kong, Cambodia, gained about 30,000 square metres.

However, excavation of the Funan Techo Canal would have a heavier impact on Thailand in economic and trade terms, because in the future it is possible that the canal could become an important trading area or economic zone, diverting cargo ship routes so they would no longer need to stop at Thai ports such as Khlong Toei Port.

At the same time, it would affect the Eastern Economic Corridor (EEC), a special economic zone on the eastern coast of the Gulf of Thailand. In addition, environmental problems would follow, with brackish water flowing into central Bangkok.

But Vietnam would be more severely affected. In addition to losing trade benefits, Cambodia’s goal in excavating the Funan Techo Canal is to create a water transport route connecting Phnom Penh to the Gulf of Thailand, reducing dependence on Vietnam by no longer having to pass through the country.

On the environmental side, it would affect the flow of water in the Mekong River and could affect water volumes and agricultural production in the Mekong Delta in southern Vietnam, because the canal excavation could divert more water from the Mekong River into the Bassac River, reducing the amount of water flowing into the Mekong Delta.

As a result, agricultural areas in southern Vietnam, especially the Mekong Delta, which is regarded as the country’s most important breadbasket and a main source of food production, such as rice and fisheries, could face immediate collapse.

If Vietnam succeeds in building the road and bridge linking Phu Quoc island, it would close off the Funan Techo Canal, preventing Cambodia from freely sending cargo ships in and out of the country as hoped, while also preventing impacts on agricultural areas.

A security source said the 1982 United Nations Convention on the Law of the Sea, or UNCLOS, as maritime law, applies to States Parties that have ratified it and must be respected. After the Thai side cancelled MOU44, Cambodia ratified UNCLOS to preserve its rights because, under the law, if a country has not ratified it, then when any rules are discussed, that country will not be invited to explain its position because it is not a member.

“When Cambodia has ratified it, Cambodia must return to maritime law. This is a positive development because, for Thailand, the law of the sea is a matter of exercising and asserting rights. All countries regard it as a point of law. Cambodia is also using the same criteria as Vietnam,” the security source said.

Therefore, the issue to watch is that Cambodia’s decision to become a party to UNCLOS indicates that Cambodia may have a card up its sleeve in its disputes with both Thailand and Vietnam. If agreements cannot be reached, the disputes are likely to move towards the international arena, including through arbitration.