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.

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....