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The Cambodian government said part of a historic 11th-century stone temple collapsed Sunday due to heavy shelling by the Thai army as the two sides battled across their disputed border for a third day.

Both countries accused each other of instigating the clashes, which continued across the darkened mountainous border for more than three hours Sunday. The extent of the damage to the Preah Vihear temple, a UNESCO World Heritage Site, was not immediately clear.

A Thai army spokesman said about 10 soldiers were wounded in Sunday night's fighting. Cambodian Prime Minister Hun Sen said the clashes Sunday resulted in "more human casualties and damages" but did not elaborate.

On Sunday, Hun Sen sent a letter to the U.N. Security Council calling for an emergency meeting to help end the fighting.

At least five people have died in the border clashes — some of the fiercest in years — which erupted Friday and continued for a third straight day Sunday despite at least two cease-fires.

The crumbling stone temple, which sits several hundred feet from Thailand's eastern border with Cambodia, has fueled nationalist sentiment on both sides of the disputed frontier for decades.

In 1962, the World Court determined that the temple belongs to Cambodia, a ruling disputed by many Thais. Thai nationalists have seized on it as a domestic political issue, and the conflict has sparked sporadic, brief battles between the two neighbors over the last few years.

The lastest fighting broke out Friday in an area close to Preah Vihear, and shelling Saturday caused minor damage to the temple's facade. There were reports that Sunday's fighting had spread closer to the temple itself. There was no independent confirmation of the damage.

"A wing of our Preah Vihear Temple has collapsed as a direct result of the Thai artillery bombardment," the government quoted a Cambodian military commander based near the temple as saying. It did not say how large the wing was.

Built between the 9th and 11th centuries, Preah Vihear is dedicated to the Hindu diety Shiva and revered partly for having one of the most stunning locations of all the temples constructed during the Khmer empire — the most famous of which is Angkor Wat. It sits atop a 1,722-foot (525-meter) cliff in the Dangrek Mountains about 150 miles (240 kilometers) north of the Cambodian capital.

UNESCO calls the site "an outstanding masterpiece of Khmer architecture." The Khmer empire, which once encompassed parts of Thailand and Vietnam, shrank to the size of present-day Cambodia. The country was plunged into civil war, and the temple fell into disrepair.

Hun Sen said the latest fighting "gravely threatened peace and stability in the region."

"Thai armed forces launched a full-scale armed aggression against Cambodia, using heavy sophisticated weapons including many ... artillery shells which were fired into the temple of Preah Vihear," Hun Sen said in a letter which was read aloud on state television shortly before midnight.

Thailand accused Cambodia of firing first on Sunday and denied knowledge of damage to Preah Vihear.

"I haven't received any report about the damage and doubt it is true," said Thai army spokesman Col. Sansern Kaewkamnerd, contacted by telephone from Bangkok. He dismissed reports of damage to the temple as "propaganda," but said Sunday's fighting was more intense than the previous two days.

"We have to return fire to wherever the attacks come from," he said. "The prime minister has told the army to open all communication channels in case the other side wants to talk. Meanwhile, we have to take self-defense and react accordingly."

Tensions have risen in recent days because of demonstrations in the Thai capital, Bangkok, demanding that the government oust Cambodians from the area near the temple.

Thailand's Prime Minister Abhisit Vejjajiva called earlier Sunday for a peaceful solution to the border dispute, but warned that Thai soldiers would defend national sovereignty if attacked.

"I insist that the dispute on the border issues must be solved through nonviolent means," Abhisit said in his weekly address to the nation. "Thailand never thought of invading anyone, but if our sovereignty is violated, we have to protect it ultimately."

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Associated Press writers Thanyarat Doksone, Todd Pitman and Jocelyn Gecker also contributed to this report.