There are reports of gunfire on the Thai-Cambodian border, where the countries are engaged in a prolonged border dispute.
Hundreds of soldiers from Thailand and Cambodia are believed to be in the area.
The ABC has been told that people near the Preah Vihear temple in north-western Cambodia have heard machine gun fire from within the 4.5 square kilometre disputed zone.
"Cambodian and Thai troops are opening fire at each other now," Cambodia army commander Bun Thean told news agency AFP.
A Reuters photographer at the scene confirmed that fighting had broken out, saying he had to take cover amid rocket and small arms fire.
"I heard gun-fire all over the place in this area," Photographer Chor Sokunthea said by telephone.
"One rocket flew from Thailand over my head and landed. Now you can hear the fighting. They've opened fire.
"I have to find a safe place to hide," he said, before hanging up.
A Cambodian official said Thai soldiers fired first in the exchange of gunfire.
"I can confirm there was fighting between Cambodians and Thais, the Thais fired at us first," Preab Tan, Governor of Cambodia's northern Preah Vihear province, told Reuters.
On Monday, Cambodia's Prime Minister, Hun Sen, threatened that the area would become a "life-and-death battle zone" unless Thailand pulled its soldiers out.
Thailand maintains that its soldiers were not on Cambodian land and refused to retreat.
Local villagers were evacuated from the area when tensions flared in July.
Thai Foreign Minister Sompong Amornvivat urged Thai nationals in Cambodia to leave as soon as possible.
"Thai businessmen who have no need to be in Cambodia now, please rush back to Thailand," Mr Sompong told reporters.
"We have our evacuation plan ready," he added.
- ABC/AFP/Reuters
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