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's military force is ranked 25th in the world and third in the ASEAN region after Indonesia and Vietnam. Thailand has 360,000 active troops, including 115,000 enlisted soldiers. In addition, there are 240,000 reserve troops and paramilitary forces, bringing the total number of military personnel to over 600,000.

Thailand bombed Cambodia with F-16 fighter jets on Thursday, as relations between the two countries imploded following clashes on a disputed border near the Emerald Triangle.