BANGKOK, Thailand — The U.S.-trained Thai military’s F-16 warplanes bombed Cambodia Thursday as Cambodia blasted truck-mounted Soviet-era artillery rockets across a disputed border region with its Southeast Asian rival, killing 12 Thai civilians including an 8-year-old boy.

Six armed, silvery U.S.-built F-16 jets winged above Southeast Asia’s tropical sky and dropped at least two bombs, pounding “Cambodian military targets on the ground,” Royal Thai Army spokesman Col. Ritcha Suksuwanon told reporters.

“We have used air power against military targets as planned,” Col. Ritcha said without elaborating on the sites.

Cambodia reportedly said Thailand’s F-16s dropped two bombs “where Cambodian forces are stationed to defend the nation’s territorial integrity,” but gave no details about damage or casualties.

Hours after Thursday’s bloody exchange, Cambodian Senate President Hun Sen reportedly told Thailand not to “boast of your superior military power, or think of invading,” because Cambodia was “fully prepared for combat.”

“You will face the most severe retaliation,” the Cambodian leader said. “We won’t just resist, we will strike back.”

Tens of thousands of panicked villagers on both sides of the frontier began evacuating the zones or hiding in bunkers while steel-helmeted Thai army forces in armored personnel carriers hurtled down highways toward the disputed region known as the Emerald Triangle jungle.

Both sides claim the other started the latest clash, which included Cambodia positioning Soviet-designed BM-21 artillery on trucks hidden among jungle and scrubland at the frontier and blasting ground-to-ground rockets into Thailand.

The fighting along Cambodia’s northern border with Thailand coincides with a dangerously destabilizing breakup between the two nations’ dynastic, billionaire, ruling families after decades of tight interlocking personal relations.

Both countries’ authoritarian de facto leaders and former prime ministers — Thailand’s Thaksin Shinawatra and Hun Sen in Cambodia — and their families are openly warring against each other over public betrayals, lies, threats and leaked confidential audio recordings.

During Thursday’s firefight, Cambodia unleashed a morning barrage at various sites, using truck-mounted 122mm BM-21 multiple rocket launchers that blasted unguided rockets into a wide zone.

The 12 civilian fatalities came when rockets hit a gasoline station, a hospital and other sites, according to Thai army reports.

“The Royal Thai Army strongly condemns the violent actions targeting civilians by Cambodian forces,” the army said.

“Thailand stands ready to defend its sovereignty and protect its people from such inhumane acts.”