A Thai F-16 fighter jet carried out an airstrike in Cambodia, both countries confirmed, as weeks of growing tension over a border dispute turned into violent clashes on Thursday, killing at least 12 people, including 11 civilians.

Thailand’s military said that one of six F-16 jets stationed along the disputed border entered Cambodian airspace and struck a military target. Both nations blamed each other for starting the fighting early Thursday.

"We have employed air power against military objectives as scheduled," Thai army deputy spokesperson Richa Suksuwanon said in a statement to media representatives. Thailand also closed its border with Cambodia.

Cambodia’s defence ministry said the jets dropped two bombs on a road and stated that it "strongly condemns the reckless and brutal military aggression of the Kingdom of Thailand against the sovereignty and territorial integrity of Cambodia".

These confrontations followed Thailand’s decision on Wednesday to recall its ambassador from Cambodia and expel Cambodia’s ambassador from Bangkok. This came after a second Thai soldier in one week lost a limb to a landmine, which Thailand claimed had been recently planted in the disputed zone.

Thailand’s health minister reported that 11 civilians, including one child, and one soldier were killed due to artillery fire from the Cambodian military, and 24 civilians, along with seven soldiers, were wounded. Cambodia did not immediately report any casualties.

"The Thai Army denounces Cambodia for resorting to weapons to harm civilians in Thailand. Thailand is prepared to defend sovereignty and our people from inhumane action," the military said in a statement.

Malaysia’s Prime Minister Anwar Ibrahim, who currently chairs ASEAN, where both countries are members, called for calm and said he would speak to leaders on both sides to help settle the conflict peacefully. China also expressed concern and offered to help reduce tensions.

Thai citizens, including children and the elderly, fled to shelters made of concrete and reinforced with tyres and sandbags in Surin province, which borders Cambodia.

"How many times have they fired? It's infinite," an unnamed woman said to the Thai Public Broadcasting Service (TPBS) while taking shelter, as gunfire and explosions were heard in the background.

Cambodia’s foreign ministry described the Thai airstrikes as "unprovoked" and urged Thailand to withdraw its troops and "desist from any further provocative actions which could aggravate the situation."

Thailand and Cambodia have argued over sections of their 817-km-long shared border for more than a hundred years. Many parts remain unmarked, and this has led to repeated skirmishes and deaths, including during a week-long artillery exchange in 2011.

Tensions rose in May after a Cambodian soldier was killed in a short gunfight. That incident sparked a major diplomatic fallout, which has now escalated into full-blown armed clashes.

The latest fighting began early Thursday near the disputed Ta Moan Thom temple along the border, about 360 km east of Bangkok.

Thailand’s Health Minister Somsak Thepsuthin told journalists that casualties occurred across three border provinces and included an 8-year-old boy from Surin.

He also said that Cambodian shelling hit a hospital in Surin province and noted that this could be treated as a war crime.

"Artillery shell dropped on houses," commented Sutthirot Charoenthanasak, chief of Kabcheing district in Surin province, in a comment to Reuters. He further remarked that 40,000 civilians had been moved from 86 border villages. "Two deaths have occurred," he commented.

Thick black smoke was seen rising from a gas station in Sisaket province, as firefighters worked to control the flames.

Eight people were killed and 15 injured in Sisaket, and one more person died in Ubon Ratchathani, another border province, the health minister said.

The Thai military said that Cambodia first used a surveillance drone, then sent troops armed with heavy weapons, including rocket launchers, near the Ta Moan Thom temple.

A spokesperson for Cambodia’s defence ministry said Thai soldiers had crossed the border without cause and that Cambodian forces responded only in self-defence.

Thailand’s acting Prime Minister Phumtham Wechayachai said the situation was fragile.

"We have to be cautious," he said. "We will abide by international law."

A recent attempt by Thailand’s former Prime Minister Paetongtarn Shinawatra to ease tensions by speaking with Cambodia’s former Prime Minister Hun Sen led to political controversy in Thailand after the conversation was leaked, resulting in her suspension by a court.

Hun Sen, in a Facebook post, claimed that two Cambodian provinces had been hit by Thai shelling.

Thailand has also accused Cambodia of planting new landmines in the disputed area, which injured three Thai soldiers. Cambodia denied the accusation, saying the soldiers had wandered off agreed paths and triggered leftover mines from earlier conflicts.

According to organisations that work on demining, Cambodia still has millions of landmines from its civil war.

Thailand, however, insists that the landmines were recently placed at the border, a claim Cambodia strongly rejects.