Thailand warned its conflict with neighboring Cambodia could “potentially develop into a war” as troops used rockets and artillery to shell targets along their contested border for a second

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.

More than 100,000 people have been displaced in the deadliest conflict between the two nations in 14 years. Thailand’s acting prime minister warned on Friday that clashes with Cambodia over a border dispute “could develop into war” as the deadliest fighting between the two countries in years extended into a second day.