Thailand launched air strikes on its neighbour Cambodia, with both sides trading blame for renewed fighting on their disputed border that has killed four Cambodian civilians and a Thai soldier.
Around 35,000 people in Thailand have been evacuated from border areas, the country's Second Army Region said.
Five days of combat between Thailand and Cambodia this summer killed 43 people and displaced around 300,000 on both sides of the border before a truce took effect.
However, last month Thailand paused a follow-on deal backed by US President Donald Trump to wind down hostilities, saying a landmine blast at the border had wounded several soldiers.
Since then, Cambodian and Thai officials have reported sporadic skirmishes along their frontier, which reignited yesterday and this morning,
Cambodia's information minister Neth Pheaktra said at least four Cambodian civilians were killed by Thai shelling in the border provinces of Preah Vihear and Oddar Meanchey.
Around ten other civilians were wounded, including a Cambodian journalist who was hit by shrapnel from a Thai rocket, Neth Pheaktra said.
The Thai army said one soldier was killed and 18 others were wounded since fresh fighting began yesterday.
Farmer Pannarat Woratham, who lives just a few kilometres from the border in Thailand's Surin province, said she fled in the afternoon to a temple sheltering the displaced.
It was the second time the 59-year-old had evacuated since late July when open combat was waged with fighter jets, missile strikes and ground troops, killing both civilians and soldiers. "Of course many of us thought the conflict was finally over. It shouldn't have happened again like this," she told AFP.
On the other side of the border, Cambodian Hul Malis said the Thai military entered the border village of Prey Chan, in northwestern Banteay Meanchey province, on this afternoon just 20 minutes after she fled.
"They came in with tanks," Hul Malis said other residents told her.
"I am so scared and I am running away" to the provincial capital, she added.
Prey Chan was the site of a stand-off in September between several hundred Cambodian demonstrators and Thai forces who fired rubber bullets and tear gas at them.
Both countries claim the area as their own territory.
Neth Pheaktra, the minister, told reporters at least 1,157 families had evacuated to safety in Oddar Meanchey alone.
The conflict centres on a century-old disagreement over borders mapped during France's colonial rule in the region, with both sides claiming a number of boundary temples.
Both sides reported a brief skirmish yesterday, which Thailand's military had said left two soldiers wounded.
But the fighting escalated early this morning.
Thai army spokesman Winthai Suvaree said it had launched air strikes against Cambodia in an act of self-defence.
"The Thai air power is being used only against Cambodian military targets," Mr Suvaree said at a press conference.
"The air strikes are highly precise and aimed solely at military objectives along the clash line, with no impact on civilians."
Cambodia's defence ministry spokeswoman Maly Socheata said Thai forces had attacked Cambodian troops in Preah Vihear and Oddar Meanchey, accusing Thailand of "firing multiple shots with tanks" near centuries-old temples.
Maly Socheata later told reporters that the Thai military attacked Cambodian forces in Preah Vihear with an F-16 jet.
In an evening press briefing, she said the Thai miliary had not ceased their attack and expanded the battle along the border.
She maintained that Cambodia has not retaliated.
The Thai army, meanwhile, has accused Cambodian forces of firing BM-21 rockets towards civilian areas in Buriram province, with no casualties reported.
The United States, China and Malaysia, as chair of the regional bloc ASEAN, brokered the cessation of fighting in July.
In October, Mr Trump co-signed a follow-on joint declaration, touting new trade deals with Thailand and Cambodia after they agreed to prolong their ceasefire.
But Thailand suspended the agreement the following month, and the two sides then traded accusations of renewed clashes in which Cambodia said a civilian was killed.
Malaysian Prime Minister Anwar Ibrahim urged both sides to halt the fighting and utilise diplomacy.
Asked about Mr Trump's intervention and Anwar's call for restraint, Thai Prime Minister Anutin Charnvirakul told reporters in Bangkok that no one should tell his country to stop, adding "we're long past that point".
"If you want things to stop, tell the aggressor to stop," he said.
