U.S.-backed talks to end the border war, in which militaries have killed dozens of people and displaced hundreds of thousands, began on Monday in Malaysia.

The leaders of Cambodia and Thailand agreed to a ceasefire in a bid to bring an end to their deadliest conflict in more than a decade.

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.