“We have at least six people killed, three of them soldiers,” an official said speaking on condition of anonymity, with “other people injured,” including civilians.
The bomb went off in Bahsas area of Tripoli where the army bus with 24 people on board was transiting on its way to Beirut.
Police immediately cordoned off the area as forensic experts began gathering evidence. Media and relatives of the victims were kept away.
At the moment no one has yet claimed responsibility for the attack.
The blast shattered windows of several cars parked in the area.
The attack is the second bombing to target Lebanese troops in Tripoli in less than two months, an area where peace and security have always been fragile.
On 13 August, 18 soldiers and civilians were killed by a roadside bomb packed with nuts and bolts near a bus carrying troops in Tripoli.
Tripoli has been rocked by sectarian fighting between pro-government Sunni fighters and pro-Syria Shia militias.
At the end of June clashes between pro-Hizbollah activists and Sunni government supporters broke out with tens of people killed or injured.
Local witnesses said the clashes were sparked by an attack by an Alawi group (to which Syrian President Assad belongs) with hand grenades and rocket launcher against an area that backs the Mustqbal party led by anti-Syrian majority leader Saad Hariri.
Syria recently deployed 10,000 soldiers along the Lebanese border. For Damascus the move was designed to counter smuggling and protect “domestic security”. Syrian troops are set to remain within Syria’s border and not cross into Lebanon.
Such precautions seem not very helpful since a bomb exploded in the Syrian capital last Saturday along the road that leads to the airport, killing 17 and injuring 14.