About 70 Thais have been at the 13th Century Ta Moan temple complex since Thursday, the Cambodians say.
The Thai foreign ministry has denied any troops have moved into the area.
The two nations have for weeks been locked in a military stand-off over disputed land further east, around the ancient Preah Vihear temples.
High-level diplomacy has been taking place to resolve the Preah Vihear dispute, which revolves around border areas drawn up by French cartographers a century ago.
And an agreement was reached, following a meeting between foreign ministers from both countries, to reduce troop numbers near the temple.
But Maj Sim Sokha, a Cambodian border-protection officer, told reporters on Sunday that Thai soldiers had been making moves hundreds of miles west of Preah Vihear.
He said the Thai troops had been deployed in an 80m (262ft) radius around the Ta Moan Thom temple ground, and had prevented Cambodian troops from entering.
He said about 40 Cambodian soldiers were in close proximity to the Thai troops, but had been ordered to exercise restraint while the government tries to resolve the issue with Thailand.
“[Thai troops] said they will pull back only when the issue near Preah Vihear temple is resolved,” he told the Associated Press by telephone.