Long-festering tensions over border territory have escalated into armed conflict between Cambodia and Thailand, leading to dozens of deaths on both sides and displacing tens of thousands of people.

The Thai warning came as the U.S. expressed its grave concern about the most serious fighting in years between the Southeast Asian neighbors. The U.S. and China compete for influence in fast-growing Southeast Asia, and both have good relations with Thailand and Cambodia.

Thailand and Cambodia traded accusations Saturday of fresh attacks as deadly border clashes entered a third day, leaving at least 33 people dead and more than 168,000 displaced, as international pressure mounted on both sides to reach a ceasefire.

China has dismissed allegations that it was supplying weapons to Cambodia amid its ongoing border conflict with Thailand that has led to at least 33 people dying and hundreds of thousands displaced.

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

Thailand and Cambodia exchanged heavy artillery fire for a second day on Friday as border fighting intensified and spread, while Cambodia's leader said Thailand had agreed to a Malaysian ceasefire proposal but then backed down.