The neighbours are locked in a bitter spat over an area known as the Emerald Triangle, where the borders of both countries and Laos meet, and which is home to several ancient temples.

The squabble has dragged on for decades, flaring into bloody military clashes more than 15 years ago and again in May, when a Cambodian soldier was killed in a firefight.

The conflict blazed up on Thursday, with Cambodia firing rockets and artillery shells into Thailand and the Thai military scrambling F-16 jets to carry out air strikes.

The Thai ministry of public health said at least 11 civilians had been killed, most of them in a rocket strike near a petrol station in Sisaket province.

Footage from the scene showed smoke pouring from the roof of a convenience store attached to the petrol station. Provincial officials said most of the dead were students inside the shop when the attack happened.

Six Thai air force jets were deployed from Ubon Ratchathani province, hitting two “Cambodian military targets on the ground”, according to Thai military deputy spokesperson Ritcha Suksuwanon.

Both sides blamed the other for starting the fighting, which erupted near two temples on the border between the Thai province of Surin and Cambodia’s Oddar Meanchey.

Cambodian defence ministry spokeswoman Maly Socheata said in a statement that Thai troops launched an “armed assault on Cambodian forces”.

“In response, the Cambodian armed forces exercised their legitimate right to self-defence, in full accordance with international law, to repel the Thai incursion and protect Cambodia’s sovereignty and territorial integrity,” she said.

Cambodian Prime Minister Hun Manet requested an urgent meeting of the UN Security Council to address what his foreign ministry labelled “unprovoked military aggression”.

Thailand’s government spokesman, meanwhile, accused Cambodia of being “inhumane, brutal and war-hungry”, and Bangkok’s foreign ministry said all border crossings had been shut and nearby residents evacuated.

The Thai military blamed Cambodian soldiers for firing first, and later accused them of a “targeted attack on civilians”, saying two BM-21 rockets had hit a community in Surin’s Kap Choeng district, wounding three people.

According to the Thai military, the clashes began around 7:35 am (0035 GMT) when a unit guarding Ta Muen temple heard a Cambodian drone overhead.

Later, six armed Cambodian soldiers, including one carrying a rocket-propelled grenade launcher, approached a barbed-wired fence in front of the Thai post, the army said.

Around 8:20 am, Cambodian forces opened fire toward the eastern side of the temple, about 200 metres from the Thai base.

Thailand’s acting Prime Minister Phumtham Wechayachai said “the situation requires careful handling, and we must act in accordance with international law”.

“We will do our best to protect our sovereignty,” he said.

In a Facebook post, Thailand’s embassy in Phnom Penh urged its nationals to leave Cambodia “as soon as possible” unless they had urgent reasons to remain.

China, a close ally of Cambodia, said it was “deeply concerned” about the clashes, calling for dialogue — while also urging its citizens in Cambodia to avoid the country’s frontier with Thailand.

Long-running row

The violence came hours after Thailand expelled the Cambodian ambassador and recalled its own envoy in protest after five members of a Thai military patrol were wounded by a landmine.

On Thursday morning, Cambodia announced it was downgrading ties to “the lowest level”, pulling out all but one of its diplomats and expelling their Thai equivalents from Phnom Penh.

Recent weeks have seen a series of tit-for-tat swipes by both sides, with Thailand restricting border crossings and Cambodia halting certain imports.

The border row also kicked off a domestic political crisis in Thailand, where prime minister Paetongtarn Shinawatra has been suspended from office pending an ethics probe over her conduct.

A diplomatic call between Paetongtarn and Hun Sen, Cambodia’s former longtime ruler and father of Hun Manet, was leaked from the Cambodian side, sparking a judicial investigation.

Malaysian Prime Minister Anwar Ibrahim called on both sides to “stand down” and start talks.

Malaysia currently chairs the Association of Southeast Asian Nations (ASEAN), of which both Thailand and Cambodia are members.