Thailand and Cambodia traded accusations of fresh attacks as deadly border clashes entered a third day, leaving at least 33 people dead and more than 168,000 displaced.
International pressure is mounting on both sides to reach a ceasefire.
Artillery fire and gunshots were reported near several border villages, expanding the area of the fighting that flared again Thursday after a land mine explosion along the border wounded five Thai soldiers.
Cambodian and Thai officials claimed to have acted in retaliation.
Both countries recalled their ambassadors and Thailand closed its north-eastern border crossings with Cambodia.
Cambodian authorities reported 12 new deaths on Saturday, bringing its toll to 13, while Thai officials said a soldier was killed, raising the deaths to 20, mostly civilians.
The regional bloc, the Association of Southeast Asian Nations, or ASEAN, is under growing pressure to defuse the situation between its two members.
During an emergency meeting on Friday, members of the UN Security Council called for de-escalation and urged ASEAN to mediate a peaceful solution.
The 800-kilometer (500-mile) frontier between Thailand and Cambodia has been disputed for decades, but past confrontations have been limited and brief.
The current tensions broke out in May when a Cambodian soldier was killed in a confrontation that created a diplomatic rift and roiled Thailand’s domestic politics.
Cambodia’s Defence Ministry condemned what it said was an expanded Thai offensive early on Saturday after five heavy artillery shells were fired into multiple locations in the province of Pursat, calling the attack an “unprovoked and premeditated act of aggression”.
Ministry spokeswoman Lieutenant general Maly Socheata said tensions flared in the province of Koh Kong, where four Thai naval vessels were reportedly stationed offshore and four others en route.
She said the naval deployment was an “act of aggression” that risked further escalation.
Lt gen Socheata said seven civilians and five soldiers were killed in two days of fighting.
Earlier, one man was reported dead after a pagoda he was hiding under was hit by Thai rockets.
The Thai army had denied targeting Cambodian civilian sites and accused Phnom Penh of using “human shields” by positioning their weapons near residential areas.
Meanwhile, Thailand’s navy, in a statement on Saturday, accused Cambodian forces of initiating a new attack in the province of Trat, saying Thai forces responded swiftly and “successfully pushed back the Cambodian incursion at three key points”, warning that “aggression will not be tolerated”.
Thai authorities also alleged several Cambodian artillery shells had landed across the border in Laos, damaging homes and property.
Lao officials have not publicly responded to the claim.
The conflict has so far left thousands displaced.
Cambodia’s information minister Neth Pheaktra said on Saturday the clashes had forced 10,865 Cambodian families, or 37,635 people, in three border provinces to evacuate to safe locations, while Thai officials said more than 131,000 people had fled their border villages.
