Thailand's military reported clashes from before dawn in the Ubon Ratchathani and Surin provinces and said Cambodia had used artillery and Russian-made BM-21 rocket systems. Authorities said 100,000 people had been evacuated from conflict areas on the Thai side.
"Cambodian forces have conducted sustained bombardment utilising heavy weapons, field artillery and BM-21 rocket systems," the Thai military said in a statement.
"Thai forces have responded with appropriate supporting fire in accordance with the tactical situation."
Both sides blamed each other for starting the conflict on Thursday at a disputed border area, which quickly escalated from small arms fire to heavy shelling in at least six locations 209 km apart along a frontier where sovereignty has been disputed for more than a century.
Reuters journalists in Surin province reported hearing intermittent bursts of explosions on Friday, amid a heavy presence of armed Thai soldiers along roads and gas stations in the largely agrarian area.
A Thai military convoy, including around a dozen trucks, armoured vehicles and tanks, cut across provincial roads ringed by paddy fields and moved toward the border.
The fighting erupted on Thursday just hours after Thailand recalled its ambassador to Phnom Penh the previous night and expelled Cambodia's envoy, in response to a second Thai soldier losing a limb to a landmine that Bangkok alleged had been laid recently by rival troops.
Cambodia has dismissed that as baseless.
The Thai death toll rose to 15 as of early Friday, 14 of them civilians, according to the health ministry. It said 46 people were wounded, including 15 soldiers.
Cambodia's national government has not provided details of any casualties or evacuations of civilians. A government spokesperson did not immediately respond to a request for comment on the latest clashes.
However Meth Meas Pheakdey, spokesperson for the provincial administration of Cambodia's Oddar Meanchey province, said one civilian had been killed and five were wounded, with 1,500 families evacuated.
Thailand had positioned six F-16 fighter jets on Thursday in a rare combat deployment, one of which was mobilised to strike a Cambodian military target, among measures Cambodia called "reckless and brutal military aggression".
Cambodia's Defence Ministry said the Thai jets dropped bombs that damaged the ancient Preah Vihear temple, which has been a site of past conflicts between the two countries.
The country's Culture Ministry said Cambodia would pursue justice under international law, since the temple was declared a World Heritage site by UNESCO, the UN's cultural organisation, and is a "historical legacy of the Cambodian people."
Thailand's use of an F-16 underlines its military advantage over Cambodia, which has no fighter aircraft and significantly less defence hardware and personnel, according to the London-based International Institute for Strategic Studies.
Malaysian Prime Minister Anwar Ibrahim, the chair of the Association of Southeast Asian Nations, of which Thailand and Cambodia are members, said he had spoken to leaders of both countries and urged them to find a peaceful way out.
"I welcome the positive signals and willingness shown by both Bangkok and Phnom Penh to consider this path forward. Malaysia stands ready to assist and facilitate this process in the spirit of ASEAN unity and shared responsibility," he said in a social media post late on Thursday.
UN Secretary-General Antonio Guterres urged both sides "to exercise maximum restraint and address any issues through dialogue," according to UN deputy spokesman Farhan Haq.
The United States, a long-time treaty ally of Thailand, called for an "immediate cessation of hostilities, protection of civilians and a peaceful resolution."
