Thai F-16 fighter jets bombed Cambodian targets on Monday, as a decades-old border dispute that President Donald Trump claimed to have ended erupted in a new salvo of fighting that killed at least five people and displaced hundreds of thousands.

Officials in Thailand and Cambodia each accused the other side of firing first, just as they had in July, when a deadly armed conflict between them raged for five days.

With the airstrikes, the Thai military said it was responding to an attack by Cambodia that killed at least one Thai soldier and injured eight others in a border province earlier in the day. The Thai army described one of its targets as a casino near the border that Cambodia used as a base for drones.

Cambodian officials said four civilians had been killed and 9 others injured in two border provinces. It was not immediately clear if the airstrikes had caused the casualties.

Hun Sen, the de facto leader of Cambodia, urged restraint. “All frontline forces have to remain patient,” he wrote on his Facebook page on Monday. “The redline of responding has been already set,” he said, without elaborating.

Thailand’s Prime Minister, Anutin Charnvirakul, told reporters in Bangkok that talks were futile because of Cambodia’s actions and that "Cambodia must comply to our conditions in order to stop the fight".

“It’s too late. We’ve been patient,” Anutin said. When asked about the agreement to resolve the dispute signed in Kuala Lumpur, Malaysia, in October, with Trump in attendance, he said: “I don’t remember that anymore.”

Chaipruak Doungprapat, the Thai army chief, said his goal was to "render Cambodia incapable of military action for a long time".

Monday’s attacks followed an exchange of fire the previous day that did not appear to be deadly. Still on Sunday, officials ordered residents in four Thai provinces along the border — Buriram, Surin, Sisaket and Ubon Ratchathani — to evacuate to shelters. The Thai military said that roughly 438,000 people had done so. Cambodian officials said on Monday that tens of thousands on their side had moved away from the border.

Malaysia’s leader, Anwar Ibrahim, who with Trump brokered the ceasefire in July, called on both Thailand and Cambodia to exercise restraint.

“Our region cannot afford to see longstanding disputes slip into cycles of confrontation,” Anwar said in a statement.

Tensions had ratcheted up on Sunday, when Thailand said that Cambodian troops had opened fire in the Thai province of Sisaket, prompting Thai forces to respond. Two Thai soldiers were injured, the Thai authorities said.

Cambodia’s defence ministry accused Thailand of firing first, into the Cambodian province of Preah Vihear, using handguns, B40 rocket launchers and 60 millimeter mortars. It said Cambodian forces contacted the Thais and demanded an immediate halt to the firing and that Cambodia had not fired back. The Thai military, it said, then stopped firing within 15 minutes.

The next day, Thai army said that Cambodia opened fire with small arms early in the morning in the Nam Yuen district of Ubon Ratchathani Province, killing at least one soldier and injuring eight others.

Later, it said that Cambodian forces had fired rounds from Soviet-era BM-21 multiple barrel rocket launchers towards Thai civilian areas in Buriram province, prompting the Thai airstrikes. It said there were no reports of casualties from the rocket attack.

New York Times News Service