SURIN, Thailand — Thai and Cambodian soldiers clashed along the border between their countries on Thursday in a major escalation that left at least 14 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, Thai Defense Ministry spokesperson Surasant Kongsiri said, a day after a land mine explosion along the border wounded five Thai soldiers and led Bangkok to withdraw its ambassador from Cambodia and expel Cambodia’s envoy to Thailand.

It was a rare instance of military conflict between member countries of the Association of Southeast Asian Nation, though Thailand has tangled with Cambodia before over the border and has had sporadic skirmishes with western neighbor Myanmar.

U.N. Secretary-General Antonio Guterres urged both sides “to exercise maximum restraint and address any issues through dialogue,” according to U.N. deputy spokesman Farhan Haq.

Each side blaimes the other

Thailand and Cambodia blamed each other for Thursday’s clashes, alleging that civilians were being targeted.

In Bangkok, the Public Health Ministry said a Thai soldier and 13 civilians, including children, were killed while 14 soldiers and 32 other civilians were injured. Public Health Minister Somsak Thepsuthin condemned what he said were the attacks on civilians and a hospital as 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 the fighting affected four provinces. The Interior Ministry was ordered to evacuate people at least 50 kilometers (30 miles) from the border.

In Cambodia, several hundred villagers moved from their homes near the border to about 30 kilometers (18 miles) deeper inside Oddar Meanchey province. Many made the journey with entire families and most of their possessions on home-made tractors, before settling down with hammocks and makeshift shelters.

From the encampment near the town of Samrong, a 45-year-old mother of four, Tep Savouen, said it all started about 8 a.m.

“Suddenly I heard a loud noise,” she told The Associated Press. “My son told me it might be thunder and I thought ‘Is it thunder or is it loud, more like a gun?’ At that time I was very scared.”

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

Thai Foreign Ministry spokesperson Nikorndej Balankura said the 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 of Phnom Penh, Defense Ministry spokesperson Lt. Gen. Maly Socheata said his country deployed armed force because “it has no choice but to defend its territory against Thai threats.” The spokesperson insisted the Cambodian “attacks are focused on the military places, not on any other place.”

Cambodian Prime Minister Hun Manet wrote to the U.N. Security Council asking for an urgent meeting “to stop Thailand’s aggression.” None was immediately scheduled, but diplomats at U.N. headquarters said there could be one on Friday.

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 any Thai nationals seeking to return home from Cambodia.

A long-standing border problem

The two Southeast Asian neighbors have long had border disputes, which periodically flare along their 800-kilometer (500-mile) frontier and usually result in brief confrontations, only rarely involving the use of weapons. The last major combat over the issue was in 2011, leaving 20 dead.

However, relations sharply deteriorated since a May confrontation killed a Cambodian soldier. Thursday’s clashes were unusually big in intensity.

The first clash on 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, prompting 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 with airstrikes to truck-mounted rockets launched by Cambodia.

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

Cambodia’s Defense Ministry said the Thai jets dropped bombs on a road near the ancient Preah Vihear temple, which has been a site of past conflicts between the two 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 diplomatic uproar

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 earlier this month over a phone call with Cambodia’s former Prime Minister Hun Sen, still 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.

Associated Press writers Sopheng Cheang ireported from Samroing, Cambodia, and Edith Lederer in New York contributed to this report.