At least 12 people were killed after Thai and Cambodian troops clashed on Thursday in a disputed area of their border, with both sides accusing each other of initiating the latest clash, officials and local media reported.
As many as 11 Thai civilians and one soldier are among the deceased, Thailand's health minister, Somsak Thepsuthin, confirmed.
Thepsuthin told reporters that Cambodia's actions, including an attack on a hospital, should be considered war crimes.
Thailand's military has placed the number of people wounded at 14 so far.
Thailand's Education Ministry closed schools near the border in Surin’s Phanom Dong Rak district. Cambodia also evacuated students and teachers from the impacted areas.
Two civilians were killed and two were injured when a Cambodian rocket targeted a border development centre in Karb Choeng district of Surin, Thailand's public broadcaster said. The victims included an eight-year-old boy.
The Thai army denounced Cambodia for firing rockets at the centre. It said that two BM-21 missiles, fired by Cambodian forces, smashed into the community centre at 9.40 am (02.40 GMT).
Thailand’s military said Cambodian troops opened fire first with heavy weapons, while Cambodia’s Ministry of National Defence said its troops acted in self-defence after coming under attack.
One civilian death was reported in the Ubon Ratchathani province and eight people, including a 15-year-old boy, and one military personnel were killed when Cambodian rockets fell into a supermarket at a petrol station in Ban Phue in Kanthararak district in Sisaket province.
The latest clash occurred a day after a Thai soldier lost his leg in a landmine explosion.
Of the six F-16 fighter jets that Thailand has readied to deploy along the disputed border, one of the aircraft fired into Cambodia and destroyed a military target, the Thai army said.
Both countries accused each other of starting the clash early on Thursday.
Cambodia on Thursday announced it is downgrading diplomatic ties with Thailand and has recalled all of its diplomatic staff from Bangkok, according to a report by Khmer Times, citing the country's Foreign Ministry.
All Cambodian diplomatic staff stationed at the Cambodian Embassy in Bangkok have been ordered to return home.
It comes a day after Thailand expelled the Cambodian ambassador and recalled its envoy from Phnom Penh over the landmine explosion incident.
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.
Fresh fighting erupted on Thursday morning near two temples on the border between the Thai province of Surin and Cambodia's Oddar Meanchey.
"Artillery shell fell on people's homes," Sutthirot Charoenthanasak, district chief of Kabcheing in Thailand's Surin province, told Reuters, describing the firing by the Cambodian side.
He said district authorities had evacuated 40,000 civilians from 86 villages near the border to safer locations.
Cambodian PM requests ‘urgent’ UNSC meeting
Cambodian Prime Minister Hun Manet on Thursday requested the UN Security Council convene an "urgent meeting".
"Considering the recent extremely grave aggressions by Thailand, which have gravely threatened peace and stability in the region, I earnestly request you to convene an urgent meeting of the Security Council to stop Thailand's aggression," Hun Manet wrote in a letter addressed to the sitting UN Security Council president Asim Iftikhar Ahmad.
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, Cambodia must 'stand down' over border clash: Malaysian PM
Malaysian Prime Minister Anwar Ibrahim on Thursday called on Thailand and Cambodia to "stand down" following the deadly border clashes between the two Southeast Asian neighbours.
"The least we can expect is for them to stand down and hopefully try to enter into negotiation," said Anwar, whose country currently chairs the Association of Southeast Asian Nations (ASEAN) which includes Phnom Penh and Bangkok.
China expresses ‘deep concern’
Separately, Chinese Foreign Ministry spokesman Lin Jian expressed "deep concern" over the conflict and urged dialogue and consultation to solve the border dispute.
Lin reiterated China's "just and impartial stance" and said Beijing will continue to "promote talks for peace" and "play a constructive role" in de-escalation between Thailand and Cambodia.
