Clashes between Thai and Cambodian troops at their disputed border have killed twelve people, Thai authorities say.

The fighting marks an escalation of the dispute between the two South East Asian neighbours that dates back to more than a century.

Most of the casualties were civilians and they were all from three Thai provinces, according to Thailand’s army, which also reported several people were wounded. Cambodia has yet to confirm if it suffered any casualties.

Both sides exchanged gunfire early on Thursday and claimed the other had fired the first shot. It escalated quickly, with Thailand accusing Cambodia of firing rockets and Bangkok carrying out air strikes on Cambodian military targets.

Thailand has closed its border with Cambodia, while Cambodia has downgraded its ties with Thailand which they accuse of using “excessive force”.

Both countries have asked their citizens near the border to leave the area, with Thailand evacuating 40,000 civilians to safer locations.

“[The fighting] is really serious. We’re in the middle of evacuating,” Sutian Phiwchan, a local resident of Ban Dan district in Thailand’s Buriram province near the Cambodian border, told the BBC.

Thai authorities said that a total of 11 civilians – including an eight-year-old and a 15-year-old – as well as one military personnel have been killed in the Surin, Ubon Ratchathani and Srisaket provinces.

Thailand and Cambodia have given differing versions of what happened.

Thailand’s National Security Council (NSC) claims that just after 07:30 local time (00:30GMT) on Thursday, Cambodia’s military deployed drones to conduct surveillance of Thai troops near the border.

Shortly afterwards, Cambodian military personnel carrying rocket-propelled grenades gathered near the border. Soldiers on the Thai side attempted negotiations by shouting, but were unsuccessful, the NSC spokesman said, adding that Cambodian soldiers opened fire at around 08:20, forcing the Thai side to retaliate.

Thailand has accused Cambodia of deploying heavy weapons, including BM-21 rocket launchers and artillery, causing damage to homes and public facilities including a hospital and a petrol station along the Thai side of the border.

Meanwhile Cambodia claims that Thai soldiers initiated the conflict at around 06:30, when they violated a prior agreement by advancing on a Khmer-Hindu temple near the border and placing barbed wire around its base.

Thai soldiers then deployed a drone just after 07:00, and fired shots “into the air” at around 08:30 according to Maly Socheata, a spokesperson from Cambodia’s Ministry of National Defence.

At 08:46, Thai soldiers “pre-emptively” opened fire on Cambodian troops leaving them no choice but to exercise their right to self-defence, according to the Phnom Penh Post newspaper quoting Socheata.

Socheata further accused Thailand of deploying excessive troops, using heavy weapons and carrying out air strikes on Cambodian territory.

The dispute dates back to more than a hundred years ago, when the borders of the two nations were drawn after the French occupation of Cambodia.

Things officially became hostile in 2008, when Cambodia tried to register an 11th Century temple located in the disputed area as a Unesco World Heritage Site – a move that was met with heated protest from Thailand.

There were sporadic clashes over the years that saw soldiers and civilians killed on both sides.

The latest tensions ramped up in May after a Cambodian soldier was killed in a clash. This plunged bilateral ties to their lowest point in more than a decade.

In the past two months, both countries have imposed border restrictions on one another. Cambodia banned imports from Thailand such as fruits and vegetables, and stopped importing power and internet services.

Both countries have also strengthened troop presence along the border in recent weeks.

Thailand’s acting premier Phumtham Wechayachai said that its dispute with Cambodia remains “delicate” and must be addressed with care, and in line with international law.

Cambodian Prime Minister Hun Manet said his country wants to resolve the dispute peacefully and that it has “no choice” but to “respond with armed force against armed aggression”.

While there have been serious exchanges of fire in the past, they de-escalated relatively quickly.

While it appears the fighting is unlikely to blow up into a full scale war, at the moment there is a lack of leadership in both countries with the strength and confidence to pull back from this confrontation.

Hun Manet, the son of a former strongman, does not yet really have his own authority. Hun Sen, his father, appears willing to push this conflict further in order to burnish his own nationalist credentials.

In Thailand, there is a shaky coalition government, backed by another former strongman Thaksin Shinawatra.

Thaksin believed he had a close personal relationship with Hun Sen and his family, and feels betrayed by Hun Sen’s decision to leak a private conversation which led to his daughter, Paetongtarn Shinawatra, being suspended as prime minister.

Additional reporting by May Titthara in Phnom Penh.