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 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.