Armed clashes on the remote, disputed border between Cambodia and Thailand killed at least 12 people on Thursday, in a dramatic escalation of tensions that threatens to erupt into a broader conflict.

Both sides’ forces accuse each other of opening fire Thursday morning. Cambodia then fired rockets on Thai soil, Thailand said, to which it responded with mortars. A Thai fighter jet later dropped two bombs near the road leading to a temple in Cambodian territory, Cambodia said.

The violence comes a day after a Thai soldier lost his leg in a landmine explosion, an incident that cratered relations between Bangkok and Phnom Penh to their lowest level in years. Thailand has since closed all border crossings with Cambodia.

Thailand’s acting prime minister, Phumtham Wechayachai, said that Cambodia fired heavy weapons into Thailand without clear targets, leading to civilian deaths, adding that Thailand will not negotiate with Cambodia until fighting along the border ends.

Cambodia’s Defense Ministry condemned what it called “brutal, barbaric, and violent military aggression,” accusing Thailand of violating international law. The ministry confirmed that a Thai F-16 had dropped two bombs on a road near the ancient Preah Vihear temple, a UNESCO world heritage site.