Clashes between Thai and Cambodian troops at their disputed border have killed at least 12 people, Thai authorities say.

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

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

Both sides exchanged gunfire early on Thursday, with each claiming the other had triggered the conflict. 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, accusing its military of using “excessive force”.

Both countries have asked their citizens near the border to leave the areas, 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 claims that it began with Cambodia’s military deploying drones to conduct surveillance of Thai troops near the border.

Cambodia says Thai soldiers initiated the conflict when they violated a prior agreement by advancing on a Khmer-Hindu temple near the border.

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 which 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 – adding, however, that it has “no choice” but to “respond with armed force against armed aggression”.

Serious exchanges of fire between the two countries have de-escalated relatively quickly.

But while it appears the current fighting is unlikely to blow up into a full-scale war, both countries currently lack leadership with enough strength and confidence to pull back from this confrontation.

Hun Manet, the son of a former strongman, does not yet have his own authority – and 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.