"In light of rapid developments and delicate circumstances that the region is going through... units tasked with monitoring and controlling the northern and eastern borders have been reinforced, in conjunction with tightening surveillance measures," the army said in a statement.

On Sunday, Syrian rebels swept into Damascus declaring they had toppled "tyrant" Assad, whose current whereabouts are unknown after he reportedly fled the country.

An AFP correspondent saw dozens of cars lining the main Masnaa crossing between the two countries as Syrian families returned home, with crowds cheering and chanting anti-Assad slogans.

Lebanese Prime Minister Najib Mikati said he discussed the situation at the Syrian border in a call with army commander Joseph Aoun and security forces chiefs.

Mikati insisted "in these calls on the priority of tightening control over the border and distancing Lebanon from the repercussions of the developments in Syria", his office said in a statement.

Authorities say Lebanon currently hosts around two million Syrians, while more than 800,000 are registered with the United Nations -- the highest number of refugees per capita in the world.

Many fled Syria after its civil war began following the repression of anti-government protests in 2011.