Thailand said it launched airstrikes into Cambodia on Monday as fresh fighting broke out in several areas along their disputed border, after both countries accused each other of breaking a ceasefire that had been brokered by U.S. President Donald Trump.

A Thai army spokesperson said at least one Thai soldier was killed and eight were wounded in the clashes that grew intense around 5.00 a m local time 2200 GMT, and added that air support was called in to strike Cambodian military positions.

Thailand's Air Force said Cambodia had moved heavy weapons, repositioned combat units and organised support teams in a way that could push the conflict further.

"These developments prompted the use of air power to deter and reduce Cambodia's military capabilities," it said in a statement.

Cambodia's defence ministry said the Thai military launched attacks at dawn on its forces at two locations after several days of provocative actions and that Cambodian troops did not fire back.

Cambodia's former longtime leader Hun Sen, the father of current premier Hun Manet said Thailand's military was "aggressors" trying to provoke a reaction and urged Cambodian forces to stay calm.

"The red line for responding has already been set," Hun Sen said on Facebook without giving more details. "I urge commanders at all levels to educate all officers and soldiers accordingly."

A senior provincial official said three Cambodian civilians had been seriously injured so far and Cambodia's defence ministry repeated that its forces had not retaliated.

The long-running border dispute between the two countries had turned into a five-day conflict in July before a ceasefire was arranged by Malaysian Prime Minister Anwar Ibrahim and Trump, who later also witnessed the signing of a wider peace agreement in Kuala Lumpur in October.

Anwar, who chairs the Association of Southeast Asian Nations bloc, urged both countries to show maximum restraint and to keep communication open.

"The renewed fighting risks unravelling the careful work that has gone into stabilising relations between the two neighbours," Anwar said in a post on X.

Recent decades have seen very few military clashes among Southeast Asian nations and cross-border air strikes have been even more rare.

Phichet Pholkoet who lives in Thailand's Ban Kruat district near the border said he had been hearing gunfire since early Monday.

"It startled me. The explosions were very clear. Boom boom!" he said by phone. "I could hear everything clearly. Some are heavy artillery, some are small arms."

The Thai military said more than 385000 civilians in four border districts were being evacuated and over 35000 had already been moved to temporary shelters.

Across the border in Cambodia, opposition politician Meach Sovannara said civilians were also leaving areas close to the fighting.

"I heard the artillery shelling," he told Reuters in an audio message from Samroang town, which is the capital of Oddar Meanchey Province bordering Thailand.

Authorities said more than 1100 families in Oddar Meanchey had been evacuated.

At least 48 people were killed, and around 300000 were displaced during the July clashes when both sides exchanged rockets and heavy artillery fire for five days.

Thailand and Cambodia have disputed control over several unmarked parts of their 817 km border for more than a century, since France first mapped it in 1907 while ruling Cambodia as a colony.

The dispute has flared up several times, including a weeklong artillery exchange in 2011, despite repeated efforts to settle the overlapping claims peacefully.

Tensions rose again in May this year when a Cambodian soldier was killed in a brief gunfire exchange and they steadily grew into diplomatic disputes and armed clashes.

Although Anwar and Trump managed to end the fighting within days and later secured a ceasefire agreement at a regional summit in October, Thailand said last month that it was suspending the truce after a landmine blast badly injured one of its soldiers.

Thailand has repeatedly said that Cambodia planted new landmines along parts of their contested border and said that these mines have seriously injured at least seven Thai soldiers since July.

Phnom Penh has denied the allegation.

Reuters reported in October that some mines found along the border were likely newly planted based on expert analysis of material provided by the Thai military.