Indonesian Foreign Minister and current ASEAN Chairman Marty Natalegawa is set to arrive in Cambodia today for talks aimed at damping down continuing tensions along the Thai border, amid calls for international mediation to resolve the dispute.

“[Natalegawa] will meet Deputy Prime Minister Hor Namhong to seek an understanding about the situation following the border clashes between Cambodia and Thailand,” Foreign Ministry spokesman Koy Kuong said.

The announcement came ahead of renewed fighting on the border last night – the third straight day of hostilities and the fiercest fighting since tensions broke out in July 2008.

On Saturday, ASEAN Secretary General Surin Pitsuwan called for both sides to halt hostilities and return to the negotiating table, reopening suggestions the dispute should be resolved through outside mediation.

“I am deeply concerned about the serious situation on the border between Thailand and Cambodia. This violent conflict must be brought under control and return to negotiating table soonest,” Surin said in a statement cited by the Bangkok Post. Surin added that he had been in touch with Cambodian and Thai leaders and that both “now welcome some form of mediation by the ASEAN leadership”.

Thailand has long opposed the issue being raised in international fora, arguing that it was possible to solve it on a bilateral basis. Ahead of yesterday’s clashes, Thai Foreign Ministry spokesman Thani Thongphakdi said Thai officials “continue to believe” that third-party mediation is not necessary.

“Both countries want to use existing bilateral mechanisms to address outstanding issues,” Thani said.

After fire fights broke out on Friday morning, Hor Namhong wrote to the United Nations Security Council accusing Thailand of “flagrant aggression” in the wake of border clashes on the weekend that left at least five people dead.

In the letter to UNSC President Maria Luiza Ribeiro Viotti dated Saturday, Hor Namhong claimed about 300 Thai troops entered Cambodian territory on Friday, forcing Cambodia to react in self-defence.

“Facing this flagrant aggression, Cambodian troops had no option, but to retaliate in self-defense and in order to safeguard Cambodia’s sovereignty and territorial integrity,” the letter stated. Thai leaders say Cambodian troops started the skirmish. In his letter, Hor Namhong claimed the clashes followed similar “acts of aggression” committed by Thailand during 2008 and 2009, which had resulted in “human casualties as well as property damage, especially to the temple of Preah Vihear”.

On Saturday, Thai Prime Minister Abhisit Vejjajiva said Bangkok planned to send letters of its own to the UNSC, UNESCO’s World Heritage Committee and foreign diplomats in order to clarify its position on the flare-up. Officials from both sides claim the letters are merely to inform the council of the situation – not a call for its intervention.

“It’s not for them to come in and mediate, but to let the UNSC know what has transpired,” said Thani.

At least seven people have been killed in border clashes since July 2008, when Preah Vihear temple was listed as a UNESCO World Heritage site.

Whether a resumption of bilateral talks can resolve ongoing tensions at the border, however, remains an open question.

“We have observed that the bilateral negotiations between Cambodia and Thailand near the Preah Vihear temple have had no progress and are completely deadlocked,” Sam Rainsy Party spokesman Yim Sovann said.

“Therefore, the opposition party will support the government’s policy for resolving the border issue with the regional and international communities.” ADDITIONAL REPORTING BY JAMES O’TOOLE