Thailand’s acting prime minister has warned 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.

More than a dozen civilians and six soldiers have been killed on the Thai side since the fighting started Thursday. In Cambodia, at least one death has been reported, and hundreds of people have fled their homes in wagons pulled by tractors. More than 131,000 people in Thailand have evacuated, according to the country’s Health Ministry.