Cambodia’s prime minister has accused Thailand of firing more than 50,000 artillery and mortar shells in more than a week of border clashes.

Prime Minister Hun Sen said Tuesday that some shells that landed in Oddar Meanchey province have not exploded and must be disarmed before civilians return to evacuated areas. Cambodia still face the problem of unexploded ordnance, especially mines, from the Vietnam War era and guerrilla wars in the decades afterward.

While much of the area remains inaccessible to journalists, witnesses did not report such intensive fire that could account for so many shells.

“It is impossible for us to fire such a number of shells,” Col. Prawit Hukaew, a spokesman for Thailand’s army in the northeast, said Wednesday. He put the number of shells fired in the hundreds.

The fighting that started April 22 has quieted in recent days. But Thailand’s state broadcaster MCOT reported a Thai soldier was killed Monday night, bringing the death toll to one Thai civilian and a total of 17 soldiers from both sides.

The conflict involves small swaths of land along the border that have been disputed for more than half a century. The latest fighting was the sixth since 2008, when Cambodia’s 11th-century Preah Vihear temple was given U.N. World Heritage status over Thailand’s objections.

Local commanders for both countries last Thursday held talks that led to a de facto cease-fire, which did not actually end the clashes but reduced their intensity. On Monday, tens of thousands of refugees from the combat zone began returning to their homes.

Hun Sen said the two sides’ regional commanders would meet to seek to implement a true cease-fire.

Hun Sen is supposed to meet Thai Prime Minister Abhisit Vejjajiva for talks on the border issue at a weekend meeting in Indonesia of the Association of Southeast Asian Nations.

The two countries have tentatively agreed to allow Indonesia observers be stationed at the border and are also seeking more lasting solutions to their dispute.

Cambodia last week asked the United Nations’ highest court to order Thailand to withdraw troops and halt military activity around a temple at the center of a decades—old border dispute that has flared into deadly military clashes.

In a request filed April 28 and made available Tuesday on the court’s website, Cambodia asked International Court of Justice judges to urgently deal with its request “because of the gravity of the situation.”

Cambodia claims that according to a 1962 ruling by the court the temple is on its territory and warns that if the intervention request is rejected and clashes continue, “the damage to the Temple of Preah Vihear, as well as irremediable losses of life and human suffering ... would become worse.”

Rulings by the court are supposed to be final and binding.

Cambodia has formally applied for an “interpretation” — a written explanation — by the court of its 1962 judgment, and argued in its written application that the court’s opinion “could then serve as a basis for a final resolution of this dispute through negotiation or any other peaceful means.”

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