But Thai officials have denied any incursion, saying the troops have been deployed on its territory to protect their country's sovereignty and control Thai activists.
Major-General Kanok Netakawesana, a Thai army field commander in the region, said on Tuesday that his troops were on Thai soil close to the disputed area.
"We are not violating the territory of Cambodia. We have every right to deploy troops here to protect our sovereignty," he said, but declined to give the number of soldiers deployed.
A Thai foreign ministry spokesman, also denied any incursion, saying the relationship between the two countries remained normal.
The ministry said in a statement late on Tuesday that the troops were ensuring that any protests by Thai activists were being done "in an orderly manner".
The confrontation comes after the United Nations' culture body Unesco declared the 1,000 year-old Preah Vihear temple a World Heritage site last week.
|A Cambodian commander said troops on both sides were on high alert [EPA]
The temple, which shares the Hindu-influenced style of the more famous Angkor complex in northwestern Cambodia, has been at the centre of a long-standing border row between the neighbours.
In 1962, the International Court of Justice awarded Preah Vihear and the land it occupies to Cambodia, a decision that still rankles many Thais.
Both countries claim land around the temple, and Thai anti-government activists have revived nationalist sentiment over the issue, blaming the government for ceding Thai sovereignty by backing Cambodia's application to Unesco.
The activists and some government officials fear the temple's new status will jeopardise their country's claims to land adjacent to the site.
The Thai troop movements followed the arrest by Cambodia of three Thai citizens for crossing the border earlier on Tuesday. The three were returned to Thailand later in the day.
The dispute has already left one Thai soldier injured, when a mine went off while he was patrolling in the area.
Stretches of the Thai-Cambodian border are still strewn with land mines sown by various sides during the 1970-75 Cambodian civil war and the guerrilla conflict that followed the fall of the Khmer Rouge in 1979.