Thailand scrambled an F-16 fighter jet to bomb targets in Cambodia on Thursday after artillery volleys from both sides killed at least 11 civilians, as border tension boiled over into rare armed conflict between the Southeast Asian countries.

Both blamed each other for starting a morning clash at a disputed area of the border, which quickly escalated from small arms fire to heavy shelling in at least six locations 209 kilometres (130 miles) apart along a frontier where sovereignty has been disputed for more than a century.

Thailand positioned six F-16 fighter jets in an uncommon combat deployment, one of which was mobilised to strike a Cambodian military target, among measures Cambodia’s foreign ministry called “reckless and brutal military aggression”.

Thailand’s military said the use of air power was to strike with precision.

The worst fighting between the countries in 13 years came after Thailand on Wednesday recalled its ambassador to Phnom Penh and expelled Cambodia’s envoy, in response to a second Thai soldier losing a limb to a landmine that Bangkok alleged had been laid recently by rival troops, an accusation Cambodia called baseless.

The United States, a long-time treaty ally of Thailand, called for an immediate end to hostilities.

“We are … gravely concerned by the escalating violence along the Thailand Cambodia border, and deeply saddened by reports of harm to civilians,” the State Department’s deputy spokesperson, Tommy Pigott, told a regular news briefing.

“The United States urges an immediate cessation of hostilities, protection of civilians and a peaceful resolution of the conflict,” he said.

The two countries have been braced for conflict since the killing of a Cambodian soldier late in May during a brief skirmish, with troops reinforced on both sides of the border amid a full-blown diplomatic crisis that brought Thailand’s fragile coalition government to the brink of collapse.

Thailand said there were 12 fatalities in three Thai provinces, 11 of them civilians, including an eight-year-old boy. Authorities said 31 people were hurt on Thursday. The number of Cambodian casualties was unclear.

“We condemn this – using heavy weapons without a clear target, outside of conflict zones… the use of force and did not adhere to international law,” Thailand’s acting Prime Minister Phumtham Wechayachai told reporters.

“We remain committed to peaceful means and there should be discussions, but what happened was a provocation and we had to defend ourselves.”

‘PREMEDITATED MILITARY AGGRESSION’

Thailand’s Health Minister Somsak Thepsuthin told reporters a hospital was hit by shelling in Surin province, an attack he said should be considered “a war crime”.

Cambodian government, defence and foreign ministry officials gave no indication of fatalities sustained or any estimate of the number of people evacuated.

In a letter to Pakistan, current president of the United Nations Security Council, Cambodia’s Prime Minister Hun Manet, urged the body to convene a meeting to stop what he called “unprovoked and premeditated military aggression” by Thailand in violation of international law.

As clashes spread to different border areas, Thai villagers including children and elderly people fled to concrete shelters fortified with sandbags and car tires.

“How many rounds have been fired? It’s countless,” an unidentified woman in Surin province told the Thai Public Broadcasting Service while hiding in the shelter as gunfire and explosions were heard in the background.

Video footage showed a plume of thick black smoke rising from a gas station in neighbouring Sisaket province, as firefighters rushed to extinguish the blaze.

Thailand has evacuated over 40,000 people from border areas, moving many to temporary shelters, where elderly people and small children gathered on floor mats as authorities prepared meals and unloaded food and bottled water from trucks.

“I’m worried about my children,” said Suphap Wongwai, an evacuee in Surin province. “My children are scared and crying.”

DECADES OF DISPUTES

Thailand and Cambodia have bickered for decades over jurisdiction of various undemarcated points along their 817 km (508-mile) land border, with ownership of the ancient Hindu temples Ta Moan Thom and the 11th century Preah Vihear central to the disputes.

Preah Vihear was awarded to Cambodia by the International Court of Justice in 1962 but tension escalated in 2008 after Cambodia attempted to list it as a UNESCO World Heritage site.

That led to skirmishes over several years and at least a dozen deaths.

Cambodia in June said it had asked the ICJ to resolve its disputes with Thailand, which says it has never recognised the court’s jurisdiction and prefers a bilateral approach.

Thailand’s use of a fighter jet underlines its military advantage over Cambodia in terms of size and range of defence hardware.

The clashes have caused jitters in the region, with the Philippines and Vietnam calling for restraint and China expressing willingness to help promote de-escalation.