EXPLAINER: Why are Thailand and Cambodia fighting over border

- 2 Southeast Asian neighbors have disputed border lying across Cambodia’s Preah Vihear province and north-eastern province of Ubon Ratchathani in Thailand

- Tensions have escalated since May 28 when a Cambodian soldier was killed allegedly in cross-border fire

- Two sides have downgraded their diplomatic ties to the lowest level in decades

The Cambodian and Thailand forces clashed along the disputed border on Thursday, firing rockets, deploying fighter jets and downgrading their diplomatic ties to the lowest level in decades.

At least 12 Thais have been killed, and another 17 injured in the fresh flare up of tensions on a century-long border dispute, which has seen frequent clashes between the two armies in recent years.

Cambodia has not released any information on the casualties.

Thailand's military deployed F-16 fighter jets allegedly hitting Cambodian military sites, while the latter responded with firing rockets, displacng hundreds of people on two sides of the border.

The two neighbors have a disputed border lying across Cambodia’s Preah Vihear province and north-eastern province of Ubon Ratchathani in Thailand.

Tensions have escalated since May 28 when a Cambodian soldier was killed allegedly in cross-border fire.

The latest escalation on Thursday was triggered by laying of landmines along the disputed border, resulting in explosions and injuries.

Border tensions also led to a political crisis in Thailand, resulting in the suspension of former Prime Minister Paetongtarn Shinawatra after a leaked recording of a private phone call between her and Hun Sen, a former prime minister of Cambodia.

In the leaked recording, Paetongtarn called Hun 'uncle,' and passed disparaging remarks about a Thai military commander.

- Colonial-era roots

Cambodia's claim on certain parts of the border banks on a 1907 map created during French colonial rule in Cambodia. However, the map's vagueness led to conflicting interpretations, and Thailand contested it.

The two sides tried to resolve the dispute diplomatically, but the issue was never settled completely, even after transnational bodies like the International Court of Justice intervened in 1962.

Clashes between the two forces began in 2008, with both sides accusing each other of starting the fighting.

The last time tensions turned deadly was seen in 2011, which killed at least 15 people and displaced tens of thousands of civilians.

The fighting focused on a jungle border area including ancient temples to which both sides laid claim.

The two nations declared a cease-fire after seven days of fighting.

- Legal interventions

Thailand claimed demarcation had not yet been completed for external parts of the area adjacent to the Preah Vihear temple, which was adjudged to be Cambodian by the ICJ with a nine to three decision in 1962.

In 2011, a UN court ordered the two neighbors to withdraw forces and establish a demilitarized zone, but the court did not rule on who would control a larger disputed territory, where troops kept clashing.

Then in November 2013, the ICJ ruled that Cambodia has sovereignty over the entire territory of the Preah Vihear temple, and that Thailand is obligated to withdraw its military personnel from the area.

Cambodian Prime Minister Hun Manet has now formally requested the UN Security Council to promptly address what he described as a “premeditated military aggression."

“It is profoundly reprehensible that this act of aggression occurs while Cambodia is actively pursuing peaceful and impartial legal avenues to resolve outstanding border issues,” Hun Manet said.He also approached the ICJ in the wake of renewed tensions on May 28.

- Latest hostilities

The border conflict deteriorated again earlier in May after the two forces engaged in another skirmish, resulting in death of a Cambodian soldier on May 28.

The soldier’s death brought the relations between the countries to the lowest point in years.

A Thai soldier lost his right leg in a land mine explosion on Wednesday, which led to the two neighbors launching fresh attacks and downgrading diplomatic relations.