LONDON — Thailand and Cambodia were engaged in armed clashes, including rocket and artillery fire, along a disputed area of their border, the U.S. State Department said in an alert.

Both countries accused the other of firing the first shots as tensions boiled over on Thursday and reports of casualties begin to emerge.

The fighting came after Thailand recalled its ambassador to Cambodia on Wednesday and said it would expel Cambodia’s envoy in Bangkok, according to Reuters.

“U.S. citizens living or traveling near the Thailand-Cambodia border should follow direction from Thai security services and consult local government authorities for the latest information,” the U.S. Embassy and Consulate in Thailand said in a security alert.

This is a developing story. Please check back for updates.

ABC News’ Will Gretsky contributed to this report.