A car bomb has struck a Hezbollah stronghold in a southern suburb of Beirut, killing five people and wounding 20 more, according to the Lebanon's Health Ministry.

As Lebanon continues to contend with the spillover from the Syrian civil war, Haret Hreik, a suburb of mixed Shia Muslims and Christians, was targeted.

Hundreds of people flooded the scene of the car bomb to help Lebanese Red Cross workers to carry the wounded to ambulances.

"A very big blast hit Haret Hreik... targeting an area that is very densely populated," Hezbollah's Al-Manar television channel said in an initial report.

In the minutes following the attack, gunshots were fired in the air to disperse the large crowd that had gathered.

The attack comes less than a week after another car bomb blast rocked the Lebanese capital, killing former minister and ambassador to the United States Mohammad Shatah and seven others.

Local television footage showed large crowds gathering around burnt-out vehicles in front of a building that had also sustained damage.

The blast may be a retaliatory act for the death of an Iranian diplomat outside the Iranian embassy in a Beirut car bomb in November which also killed 25 others.

No group has yet claimed responsibility for the attack.

More to follow.