Foreign Ministry permanent secretary Virasakdi Futrakul summoned Cambodia's charge d'affaires in Bangkok, Ouk Sopon, to take back an aide-mémoire over the border skirmish near Preah Vihear temple.
Yesterday, at about 2.30pm, Cambodian soldiers opened fire on Thai paramilitary rangers who were patrolling the area of Phu Ma Khua, a kilometre west of the temple.
The aide-mémoire said Thailand considers this intrusion and shooting as a serious violation of Thai sovereignty and territorial integrity as well as an act of aggression that is in blatant violation of international law.
"The Thai government strongly protests against the act on the part of the Cambodian soldiers," it read.
Cambodia's Foreign Ministry had earlier protested about Thailand's "invasion and shooting" near the Keo Sikha Kiri Svara Pagoda.
However, Virasakdi insisted that clashes had only taken place at Phu Ma Khua and that it was the Cambodian side that had fired first.
The permanent secretary said he interpreted the attack as a sign of pressure from Phnom Penh to have Thailand remove all its troops out of areas adjacent to Preah Vihear and the border, as well as forcing Bangkok to accept Cambodia's claimed boundary.
"Cambodia might understand that we are having domestic trouble, but we won't allow them to exploit the situation," Virasakdi said.
He also said that this might be a way for Phnom Penh to show its disappointment because Foreign Minister Sompong Amornwiwat had not come to any agreement during his visit to Cambodia on Monday.
"But we informed them in advance that the visit was merely a courtesy call," Virasakdi explained.
Cambodian Prime Minister Hun Sen presented Sompong with an ultimatum when they met in Phnom Penh on Monday, saying his troops would use force, Virasakdi said.
Military attachés from Asean countries have been invited to Thailand for a briefing on the situation, Army spokesman Sansern Kaeowkamnerd said.
The authorities are getting ready to evacuate more than a thousand Thai citizens from Cambodia and hundreds others from the border areas, the foreign minister said yesterday.
About 1,000 Thais living in Phnom Penh and 450 to 500 others in the tourism hub of Siem Reap would be evacuated by air if the conflict escalated, he promised.
"The evacuation would be made immediately we deem it necessary," Sompong told reporters. "But we hope the plan will not have to be enforced."
The number of flights from Phnom Penh and Siem Reap would have to be increased to provide seating for all, he said.