They are replacing Syrian intelligence agents who have controlled the checkpoint in the eastern Bekaa Valley for nearly three decades.
The moves come as senior Syrian and Lebanese military officers discuss details of a Syrian plan to withdraw its remaining forces from Lebanon by the end of April.
Syria said in March that it would first re-deploy its forces to the Bekaa Valley. UN envoy Terje Roed-Larsen said yesterday that Damascus has confirmed those redeployments are finished and that Syrian security offices in Beirut have been closed.
"Foreign Minister [Farouk al] Sharaa has also informed me that all Syrian troops, military assets and the intelligence apparatus will have been withdrawn fully and completely at the latest by the 30th of April 2005," Roed-Larsen said. "This commitment implies that all Syrian security forces and equipment will have been withdrawn from Lebanon in the spirit of the  Taif agreement and in full consistence with [UN] Security Council resolution 1559."
UN resolution 1559 was co-sponsored last autumn by the United States and France. It calls for the immediate withdrawal of all foreign troops from Lebanese territory.
Syrian Foreign Minister Farouk al-Sharaa stressed yesterday that by the end of April, Syria will be in full compliance:
"There are two things here," he said. "The first is [that] I agree with what Mr [Roed]-Larsen has delivered in his statement. My second notion is that Syria, in its full withdrawal, is saying that it has implemented its part of UN Security Council resolution 1559."
Nadim Shehadi, the director of the Center for Lebanese Studies at Oxford University, tells RFE/RL he has no doubt Syria's withdrawal is the result of strong international pressure since the Feb. 14 assassination of former Lebanese Prime Minister Rafik al-Hariri.
"Syria is probably in a situation which is similar to [where] Iraq was before the American invasion in March 2003," Shehadi says. "It is very keen on showing that it is complying with UN Security Council Resolution 1559 and withdrawing all of its troops from Lebanon -- including the security forces which are not mentioned in the Security Council resolution. So it is going further than what is required. It is trying to undercut any excuse that the international community or the U.S. may have to further action against Syria at a time like this."
The declared timetable means all Syrian forces will have left Lebanon before parliamentary elections. Those elections were due to take place in May. But Shehadi and other analysts say the vote may be delayed because of political turmoil since Hariri's assassination.
Pro-Syrian Prime Minister Omar Karami resigned last month amid street protests over Hariri's death, only to be reappointed by parliament. But he has been unable to form a government to organize parliamentary elections.
Opposition leaders say Karami and other Syrian-backed officials are trying to postpone the polls because they fear losing their majority in the 128-seat assembly.
Shehadi says Syria's withdrawal does not mean an end to Damascus's influence. He says Syria is capable of, in his words, "pushing certain buttons" to destabilize the situation and delay elections:
"What is happening on the ground in Beirut -- we've had four explosions in the past two weeks which are designed to destabilize the situation and create a feeling of insecurity. And we still have not had a government in Beirut," says Shehadi. "There is a political deadlock. We are probably heading toward a constitutional crisis in Lebanon -- which may not allow the elections to take place. This is part of the maneuvers that are associated with Syrian intervention still."
The United States has argued that Syria is imposing its will on Lebanon. The U.S. also has accused Syria of making a mockery of democratic principles by using its agents to threaten members of the Lebanese parliament.
France's UN ambassador has registered concerns about what Paris calls "persistent, serious interference" in the political life of Lebanon. France has said that Lebanon's future must include the "full restoration of sovereignty" -- and not an "intensification of interference."