The Thai military said eight of the casualties occurred in Si Sa Ket province, where video footage showed a plume of thick black smoke rising from a petrol station.
There was no immediate word of casualties in Cambodia.
The Cambodian government sent a letter to the United Nations Security Council on Thursday afternoon requesting an urgent meeting to bring an end to the fighting.
Unverified social media footage showed apparent chaos at a school in Cambodia’s Oddar Meanchey province, which borders Thailand, as students and teachers fled their classrooms.
The dispute over large swaths of the border is multi-generational, but the current crisis can be traced to a skirmish on May 28 that left Cambodian soldier Suon Roun dead.
Both sides blame each other for starting the fighting. A statement from Cambodia’s Foreign Affairs Ministry said Thailand’s “reckless and hostile” acts were a “blatant and serious violation of international law”.
This photo released by the Royal Thai Army shows an injured Thai soldier who stepped on a land mine being airlifted to a hospital in Ubon Ratchathani province, Thailand, on Wednesday.Credit: Royal Thai Army/AP
Cambodia’s influential former prime minister, Hun Sen, said the army had “no choice but to fight back”.
“Please, Cambodian compatriots, do not panic and buy rice and other goods to keep stock, and do not sell goods at high prices,” he said.
Thailand’s military said the fighting started near the ancient Ta Muen Thom temple complex when soldiers reported hearing a Cambodian drone circling overhead.
This photo released by the Royal Thai Army shows the scene after Cambodia fired artillery shells into a residential area at Surin Province, Thailand, on Thursday.Credit: AP
“Subsequently, Cambodian forces brought weapons into position at the front of the barbed-wire perimeter,” it said.
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“Six Cambodian personnel were observed, fully armed, including with RPG weapons, approaching the barbed-wire area in front of Thai operational base.
“Thai forces employed verbal negotiations through loudspeakers … however, at approximately 08:20 hours, Cambodian forces opened fire toward the area opposite the operational base.”
Thailand’s Foreign Ministry has called on its citizens in Cambodia to return home.
Last week, three Thai soldiers were injured after stepping on landmines in one of the border regions. On Wednesday, five more were injured in a separate explosion.
The Thais said they were planted by the Cambodians – a violation of international rules. The Cambodians said the mines were leftovers from old wars.
Hours before the fighting on Thursday, Thailand had expelled Cambodia’s ambassador and recalled its own envoy.
