BANGKOK (AP) — Thai and Cambodian soldiers clashed in several areas along their border Thursday in a major escalation of their conflict that left at least 12 people dead, mostly civilians. The two sides fired small arms, artillery and rockets, and Thailand also launched airstrikes.

Fighting took place in at least six areas along the border, Thai Defense Ministry spokesperson Surasant Kongsiri said. The trigger for the clashes was a mine explosion along the border Wednesday that wounded five Thai soldiers and led Bangkok to withdraw its ambassador to Cambodia and expel Cambodia’s.

The fighting was a rare example of open military conflict between member countries of the Association of Southeast Asian Nation. However, it is not unprecedented, as Thailand has tangled with Cambodia before over the border, and has had sporadic skirmishes with western neighbor Myanmar.

Each side claims the other started the fighting

Thailand and Cambodia blamed each other for the fighting Thursday, and alleged that civilians were being targeted.

Public Health Minister Somsak Thepsuthin said a soldier, and 11 Thai civilians, including children eight and 15 years old, were killed in the fighting, and that two dozen more people were injured. He said his agency condemned the attacks on civilians and a hospital, calling them violations of international humanitarian law and the Geneva Conventions.

"We urge the Cambodian government to immediately halt these war criminal actions, and return to respecting the principles of peaceful coexistence,” he said.

Thailand’s Acting Prime Minister Phumtham Wechayachai said four provinces were affected by the fighting, and the Interior Ministry had been ordered to evacuate people at least 50 kilometers (30 miles) from the border for their safety.

Cambodia released no details on deaths or injuries on its side.

Thai Foreign Ministry spokesperson Nikorndej Balankura said that the Thai government was “prepared to intensify our self-defense measures if Cambodia persists in its armed aggression and violations upon Thailand’s sovereignty.”

In the Cambodian capital Phnom Penh, Defense Ministry spokesperson Lt. Gen. Maly Socheata said his country employed armed force because “it has no choice but to defend its territory against Thai threats. The most important purpose is to defend itself, and the attacks are focused on the military places, not on any other place.”

Cambodian Prime Minister Hun Manet wrote to the current president of the U.N. Security Council asking for an urgent meeting “to stop Thailand’s aggression.”

Thailand also sealed all land border crossings while urging its citizens to leave Cambodia. Officials said all seven Thai airlines expressed willingness to help bring back from Cambodia any Thai nationals seeking to return home.

Conflict over the border has been a longstanding problem

The Southeast Asian neighbors have longstanding border disputes that periodically flare along their 800-kilometer (500-mile) frontier and usually result in brief confrontations, only sometimes involving the use of weapons. The last time a major armed confrontation over the issue took place was in 2011, causing 20 deaths.

But relations have deteriorated sharply since a confrontation in May that killed a Cambodian soldier, and Thursday’s clashes were far bigger in scale and intensity than usual.

The first clash Thursday morning happened in an area near the ancient Ta Muen Thom temple along the border of Thailand’s Surin province and Cambodia’s Oddar Meanchey province. It caused villagers to scurry to shelter in concrete bunkers.

The Thai army and Cambodia’s Defense Ministry each said the other side deployed drones before advancing on the other's positions and opening fire. The two sides later used heavier weaponry such as artillery, causing greater damage and casualties, and Thailand said it responded to truck-mounted rockets launched by Cambodia with airstrikes.

Thailand’s air force said it deployed F-16 jet fighters in two attacks on Cambodia. Nikorndej, the Thai Foreign Ministry spokesperson, called it “an act of self-defense” in response to the Cambodian rockets.

Cambodia’s Defense Ministry said the Thai jets dropped bombs on a road near the ancient P reah Vihear temple, which has been a site of past conflicts between the countries.

Cambodian authorities distributed photos they claimed showed damage caused there, and the country’s Culture Ministry said it would pursue justice under international law, since the temple was declared a World Heritage site by UNESCO, the U.N.’s cultural organization, and is a “historical legacy of the Cambodian people.”

A land mine explosion and diplomatic brouhaha preceded clashes

Relations frayed badly even before the clashes began. On Wednesday, Thailand withdrew its ambassador to Cambodia and expelled the Cambodian ambassador to protest the mine blast that wounded its soldiers.

Thai authorities alleged the mines were newly laid along paths that both sides had agreed were supposed to be safe. They said the mines were Russian-made and not of a type employed by Thailand’s military.

Cambodia rejected Thailand’s account as “baseless accusations,” pointing out that many unexploded mines and other ordnance are a legacy of 20th century wars and unrest.

Cambodia also downgraded diplomatic relations, recalling all Cambodian staff on Thursday from its embassy in Bangkok.

The border dispute has also roiled Thailand's domestic politics. Prime Minister Paetongtarn Shinawatra came under fire for for being too fawning in a phone call with Cambodia's former Prime Minister Hun Sen — who still is a power broker in his country — when she tried to defuse the situation. She then was suspended from office on July 1 pending an investigation into possible ethics violations over the matter.

In a prepared statement, Japanese Foreign Minister Takeshi Iwaya said Tokyo was deeply concerned over Thursday's fighting.

“A sound relationship between Cambodia and Thailand is extremely important for peace and stability in the region,” he said. “Japan urges both Cambodia and Thailand to exercise maximum restraint and strongly hopes that the tension between the two countries will be eased peacefully through dialogue.

___

Associated Press writer Sopheng Cheang in Phnom Penh, Cambodia, contributed to this report.