Foreign Affairs Minister Sihasak Phuangketkeow said bilateral talks with Cambodia on land-border issues had stalled after Phnom Penh initiated compulsory conciliation under an international treaty over a maritime boundary dispute.
Speaking in Beijing on Monday, Mr Sihasak said bilateral negotiations were no longer possible under current circumstances, although Thailand believes direct dialogue is the most appropriate approach for resolving disputes.
"At this point, the door is closed. There is no opportunity to talk about land boundaries. It will be difficult," Mr Sihasak said.
He said the Chinese government had expressed its readiness to help facilitate the restoration of relations between Thailand and Cambodia during Prime Minister Anutin Charnvirakul's official visit to China.
Mr Sihasak said Thailand appreciated Beijing's offer and maintained that any resolution should begin with bilateral negotiations.
However, he said such talks could not proceed after Cambodia chose to enter the United Nations Convention on the Law of the Sea (UNCLOS) process.
Last month, Thailand's cabinet approved the appointment of two former presidents of the International Tribunal for the Law of the Sea (ITLOS), from South Africa and Germany, to serve on Thailand's conciliation panel under UNCLOS.
Mr Sihasak was appointed as head of the negotiating team, with Songchai Chaipatiyut, Thailand's ambassador to Kuwait, serving as deputy head of the delegation.
Mr Sihasak's remarks came after Chinese President Xi Jinping urged Thailand and Cambodia to resolve their border dispute through dialogue,
According to China's Xinhua News Agency, Mr Xi said Beijing was prepared to play a "constructive role" in promoting a peaceful settlement.
