Phnom Penh (AFP) — Cambodia initiated a UN-backed dispute resolution process on Tuesday with neighbor Thailand over overlapping maritime claims in the Gulf of Thailand where undersea energy reserves are valued at around $300 billion.

The Southeast Asian nations have disputed maritime territories and the demarcation of their 800-kilometer land border for decades, a legacy of the French colonial era.

The two sides agreed to a ceasefire deal in December after two rounds of deadly border clashes, but they have repeatedly accused each other of truce violations.

The move by Cambodia to begin the mediation process, taken under the United Nations Convention on the Law of the Sea (UNCLOS), followed Thailand’s cancellation last month of a 2001 bilateral agreement on overlapping maritime claims and energy exploration.

Thai Prime Minister Anutin Charnvirakul said his government had revoked the memorandum of understanding, known as MoU 44, due to a long-standing stalemate in implementing it.

Cambodian Prime Minister Hun Manet said on state-run TVK on Tuesday that his government had submitted a formal notice to Thailand and the UN secretary-general to “begin compulsory conciliation proceedings” under UNCLOS.

“We have taken this step to protect Cambodia’s sovereignty and maritime rights in accordance with international law,” Hun Manet said

“Both Cambodia and Thailand stand to gain from a fair and lasting settlement agreed with the guidance of the international expert conciliators.”

Both countries are parties to the UN convention.

Thailand’s foreign ministry did not immediately reply to a request for comment from AFP on Tuesday.

Thailand’s energy ministry has estimated future revenues from oil and natural gas in the nations’ overlapping claims area to be worth around $300 billion.

The neighbors’ long-standing border dispute erupted into fighting in July and December, killing dozens of people and leaving several areas in border provinces claimed by both nations under Thai control.