Car bomb in Lebanon town kills three

A powerful car bomb explosion has killed at least three people and wounded 20 in downtown Hermel, a stronghold of Lebanon's Hezbollah near the border with Syria, a medic says.

The blast, which went off in front of the main government administration building in Hermel on Thursday, was the first to hit the town since the conflict in Syria erupted in March 2011, a security official said.

"The attack took place at 8.55am (1755 AEDT), just as people were going to work," the official said on condition of anonymity.

Witnesses said the attack, which damaged buildings around Hermel's main square, shocked residents in the middle of the town, a bustling commercial, residential and administrative area.

They also said the security forces deployed in the square shortly after the attack.

Though Thursday's was the first attack of its kind, the town has been hit in recent months by mortar attacks related to the war in neighbouring Syria.

The Shi'ite movement Hezbollah has since last May been openly involved in Syria's conflict, sending in elite fighters to support troops loyal to President Bashar al-Assad.

Until 2005, Lebanon had been dominated politically and militarily by Syria for 30 years.

It is still sharply divided into pro- and anti-Damascus camps, and the Syrian conflict has compounded Lebanon's sectarian and political divisions.