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.

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

The Thai prime minister's office said a Cambodian artillery shell hit a house over the border, killing one civilian and wounding three others, including a five-year-old child.

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.

"The Thai military violated the territorial integrity of the Kingdom of Cambodia by launching an armed assault on Cambodian forces stationed to defend the nation's sovereign territory," defence ministry spokeswoman Maly Socheata said in a statement.

"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."

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, approached a barbed-wired fence in front of the Thai post, the army said.

Thai soldiers shouted to warn them, the army said, but 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.

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

China also issued a warning 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.

Wechayachai said an investigation by the Thai military found evidence that Cambodia had laid new landmines in the disputed border area -- a claim denied by Phnom Penh.

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.

Last week, Hun Manet announced that Cambodia would start conscripting civilians next year, activating a long-dormant mandatory draft law.