"The U.S. already flunked the test and that should be a wakeup call," a former senior U.S. State Department official told Newsweek.

Clashes between Thai and Cambodian troops took place along their border for a second day early on Friday, with Cambodia using heavy weapons including artillery and rockets, Thailand's military said.

Cambodia rejected Thailand's account as “baseless accusations.”

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.