The action on Wednesday came hours ahead of a UN report set to accuse Damascus of arming fighters in Lebanon.
The army set up checkpoints at Sultan Yacub in the eastern Bekaa Valley, where the Syrian-backed Popular Front for the Liberation of Palestine-General Command (PFLP-GC) runs a tunnel network dug into the hills, witnesses and security sources said.
Lebanese Army commandos and tanks also deployed in force along other parts of the remote border where suspected Palestinian fighters shot and killed a civilian army contractor on Tuesday, they said.
Aljazeera's correspondent in Lebanon said that the Popular Front for the Liberation of Palestine General Command had put its forces on alert as a defensive measure, after Lebanese army had imposed an intense security ring on one of the front's headquarters in al-Sultan Yaqup area in the Lebanese Bekka.
Lebanese troops backed by tanks and armoured vehicles are heading to the scene, Aljazeera's correspondent added.
UN special envoy Terje Roed-Larsen is due to send the Security Council his latest report on Wednesday on the implementation of Resolution 1559, which demands all armed groups in Lebanon disarm and foreign troops withdraw.
Larsen(R) is due to present his
report to the UN on Wednesday
Roed-Larsen is expected to report that Syria continues to arm proxy fighters and run spies in Lebanon despite withdrawing its own troops from its neighbour in April, Lebanese official sources said.
The report would compound international pressure mounting on Syria since a UN inquiry last week implicated it in the killing of former Lebanese Prime Minister Rafiq al-Hariri in February.
About 390,000 Palestinian refugees live in 12 camps around Lebanon, which the Lebanese army does not enter.