Tensions along Cambodia's northern border with Thailand have been simmering since May, when a Cambodian soldier was killed in a clash with Thai forces.

As fighting spread to at least six areas along the arcane frontier, Thailand’s military closed crossings between the countries. The fighting spurred at least 40,000 civilians from more than 80 villages near the border to flee to makeshift bomb shelters of sandbags and car tires.

Thailand warned its conflict with neighboring Cambodia could “potentially develop into a war” as troops used rockets and artillery to shell targets along their contested border for a second day.