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.

The latest flare-up started on Thursday, with intense fighting spreading across multiple border areas. Early Saturday, Thailand’s navy joined the army in repelling what it described as incursions by Cambodian troops at three points in eastern Trat province.

Thai and Cambodian troops clashed in at least six areas along the countries’ shared 510-mile border on Thursday, killing one Thai soldier and nearly a dozen Thai civilians. The fighting—the second instance of armed confrontation between the two countries in almost two months—has sparked fears that the conflict could escalate into open warfare.

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

US President Donald Trump on Saturday announced that both the leaders from Thailand and Cambodia have agreed to meet immediately for ceasefire talks after three days of clashes

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.

Both sides have accused each other of firing the first shot as border tensions between the two South East Asian neighbours heat up.

Thai men's top seed Kunlavut Vitidsarn swept aside Malaysian Leong Jun Hao with a 23-21 21-7 win to march into the quarter-finals of the China Open on Thursday, while women's top seed An Se-young beat Sim Yu-jin 21-13 21-15 in an all-South Korean clash.