Thailand has launched airstrikes and said it has struck military targets along the border with Cambodia, as both sides accused the other of attacking first.
One Thai soldier was killed and four others wounded, according to Thai army spokesperson Major General Winthai Suvaree, who also said Cambodian troops fired first into Thai territory.
Thailand used aircraft "to strike military targets in several areas to suppress Cambodian supporting fire attacks," he added.
But Cambodian officials said the Thai military attacked their troops first.
"Cambodia urges that Thailand immediately stop all hostile activities that threaten peace and stability in the region," said Cambodian defence ministry spokesperson Maly Socheata.
She said Cambodia did not retaliate during the initial attacks on Monday.
Videos on social media appeared to show civilians evacuating from the area.
Tensions have simmered since the neighbouring countries signed a US-brokered ceasefire agreement in October, put forward by US President Donald Trump.
Their territorial disputes led to five days of combat in July that killed dozens of soldiers and civilians.
Thailand's military said the recent clashes broke out around two areas in the easternmost province of Ubon Ratchathani.
In Thailand, more than 385,000 civilians across four border districts are being evacuated, with over 35,000 already housed in temporary shelters, military officials said.
Meanwhile, the Cambodian Education Ministry said several schools along the border had been shut.
Footage posted online showed what appeared to be students hurriedly leaving classes to meet their parents.
Malaysia's Prime Minister Anwar Ibrahim urged Thailand and Cambodia to "exercise maximum restraint" and "maintain open channels of communication," he wrote in a post on X.
For more than a century, Thailand and Cambodia have contested sovereignty at points along their 508-mile (817km) land border.
Be the first to get Breaking News
Install the Sky News app for free
