Cambodia has told the UN the situation at the border is becoming "explosive", AFP news agency reports.
A Cambodian government spokesman blamed the encroachment of Thai soldiers for the fighting and said a complaint would be sent to the UN Security Council.
He said that at about 3pm, more than 300 Thai soldiers entered Cambodian territory at Wat Keo Sikha Kiri Svara adjacent to the Preah Vihear temple complex and fired mortars and artillery – 105mm, 130mm and 150mm shells – into Cambodian territory