At least one Thai soldier has been killed and four wounded in the fresh clashes that broke out around two areas in the easternmost province of Ubon Ratchathani, Thailand's military said in a statement, after its troops came under Cambodian fire.
"The Thai side has now begun using aircraft to strike military targets in several areas," the statement said.
Four Cambodian civilians have been killed and nine others injured after the fighting erupted, Cambodian Information Minister Neth Pheaktra said in a Facebook post.
Cambodia's defence ministry said the Thai military had launched dawn attacks on its forces at two locations, following days of provocative actions. It added that Cambodian troops had not retaliated.
Thailand's army said Cambodia's military had fired BM-21 rockets towards Thai civilian areas, adding there were no reports of casualties.
Thailand's Prime Minister Anutin Charnvirakul said on Monday his country did not want to see violence but the military was ready to take necessary measures to maintain the country's security and sovereignty.
The border dispute had erupted into a five-day war in July, before a ceasefire deal brokered by Malaysian Prime Minister Anwar Ibrahim and Trump, who also witnessed the signing of an expanded peace agreement between the two countries in Kuala Lumpur in October.
At least 48 people were killed and an estimated 300,000 temporarily displaced during the July clashes, with the neighbours exchanging rockets and heavy artillery fire. But following a landmine blast last month that maimed one of its soldiers, Thailand said it was halting the implementation of the ceasefire pact with Cambodia.
Cambodia's influential former leader Hun Sen, father of current premier Hun Manet, said Thailand's military were "aggressors" seeking to provoke a retaliatory response and urged Cambodian forces to exercise restraint.
"The red line for responding has already been set. I urge commanders at all levels to educate all officers and soldiers accordingly," Hun Sen said on Facebook, without elaborating.
In Thailand, more than 385,000 civilians across four border districts are being evacuated, with over 35,000 already housed in temporary shelters, the Thai military said.
Thailand and Cambodia have for more than a century contested sovereignty at undemarcated points along their 817km land border, first mapped in 1907 by France when it ruled Cambodia as a colony.
Simmering tension has occasionally exploded into skirmishes, such as a week-long artillery exchange in 2011, despite attempts to peacefully resolve overlapping claims.
