Residents on both sides of Thailand's and Cambodia's border evacuated in droves on Monday as fresh clashes erupted, killing at least five people.

Both sides have each accused the other of starting the violence, which is the most serious confrontation between the two countries since they agreed to a ceasefire in July.

Thailand's Prime Minister Anutin Charnvirakul says his country "never wanted violence" but will "use necessary means to preserve its sovereignty", while Cambodia's former leader Hun Sen accused Thai "invaders" of provoking retaliation.

Since May, escalating tensions between the neighbours have led to more than 40 deaths, as well as import bans and travel restrictions.

On Monday, the Thai army said its troops had responded to Cambodian fire in Thailand's Ubon Ratchathani Province, including by launching air strikes along the disputed border; while Phnom Penh's defence ministry said it was the Thai forces that attacked first, in Cambodia's Preah Vihear province.

At least one Thai soldier and four Cambodian civilians have been killed, and about a dozen wounded as a result of Monday's fighting, according to officials on either side.

For Thai teacher Siksaka Pongsuwan, though, there are other, hidden victims of the clashes: the children living near the border, he warns, are "losing opportunities and... precious time" compared to their peers living in relatively peaceful cities.

Children bearing the brunt

Nearly 650 schools across five Thai provinces have been ordered to shut for safety reasons, Thailand's education minister said, following the fresh tensions that have simmered since Sunday.

Meanwhile, videos on social media show chaotic scenes at schools in Cambodia's border provinces as parents rushed to take their children home.

And this is not the first time these children have had their education interrupted in recent months.

In July, in the midst of the children's examinations, five days of intense fighting broke out between the two nations.

In the aftermath, Pongsuwan's school switched to online classes, but not all students could access them - some lived in households with no internet, while iPads distributed by the schools did not reach everyone.

In Cambodia, former journalist Mech Dara shared several clips of children frantically running out of their schools on his X account.

"How many times [do these] kids have to suffer the shocking environment?" he wrote. "The nonsense fighting bring kids [a] horrible nightmare."

He also shared a picture of a boy, still in his school uniform, having some food in an underground bunker. "Why does the kid and his family have to eat their meal in the bunker...?" he wrote.

Meanwhile, Pongsuwan told the BBC that he and his neighbours are now torn about whether to evacuate - even as gunfire can be heard every now and then in his village.

"If you ask whether we're scared, yes we are... Should we leave? Will it really be safer? Or should we stay?" he tells the BBC.

A century-old dispute, reignited

The century-old border dispute between the South East Asian nations dramatically escalated with a Cambodian rocket barrage into Thailand on the morning of 24 July, followed by Thai air strikes.

Days later, Bangkok and Phnom Penh agreed to an "immediate and unconditional ceasefire" brokered by Malaysian Prime Minister Anwar Ibrahim.

In October, both sides signed a ceasefire agreement during a ceremony with US President Donald Trump in Malaysia. At the time, Trump claimed a historic achievement in ending the border conflict.

But just two weeks after that signing, Thailand said it would suspend the implementation of the agreement, after two of its soldiers were injured in a landmine explosion near the Cambodian border.

Cambodia, which nominated Trump for a Nobel peace prize for his role in brokering the ceasefire, has repeatedly claimed it is committed to the deal.

Thailand and Cambodia have been contesting territorial sovereignty along their 800km land border for more than a century, since the borders of the two nations were drawn after the French occupation of Cambodia.