Thailand launched air strikes on Cambodia on Thursday as a simmering border dispute erupted into open conflict, with soldiers on both sides trading fire – and Cambodian troops unleashing rockets that killed at least 12 Thais, most of them civilians.

Both countries traded blame for the clashes, which began at the flashpoint Ta Moan Thom temple in a disputed zone near Thailand’s eastern frontier withCambodia.

As their long shared border was closed and fighting escalated through Thursday morning, six Thai F-16 fighter jets were deployed and fired on Cambodian positions, hitting “two military targets on the ground”, according to Thai army spokesman Ritcha Suksuwanon.

Cambodia’s defence ministry condemned what it called reckless and brutal Thai military aggression, accusing its neighbour of opening fire on its troops and violating an agreement designed to de-escalate a territorial dispute that has bubbled for weeks.

In a day marked by competing narratives, the Thai army said Cambodian troops sparked the clash early on Thursday after approaching them with drones and men armed with rocket-propelled grenades. Cambodia said the Thai side had opened fire first.

As the clashes intensified, Cambodia appeared to have fired multiple salvoes of Soviet-era BM-21 rockets over the border with Thailand.