A suicide car bomb attack killed four people late Sunday in a area dominated by powerful Shiite movement Hezbollah near the Syrian border in the Bekaa Valley, a Lebanese security source told AFP.
The attack was claimed by Liwa Ahrar al-Sunna in Baalbek, an extremist Sunni Muslim group opposed to Hezbollah's involvement in the Syrian war.
The bomb killed two Hezbollah members, including local official Abdel Rahman al-Qadi, and a woman and her husband who died from their wounds shortly after the blast, the source said.
"A car bomb attack has struck the village of Al-Nabi Othman," the source said.
"The blast was carried out by a suicide attacker. Hezbollah members knew he was about to carry out the attack, and tried to stop the vehicle. That was when the attacker detonated the vehicle," he added.
Hezbollah-dominated areas in eastern Lebanon and southern Beirut have suffered a series of deadly attacks, many of them suicide car blasts, since the movement acknowledged sending fighters into Syria to support President Bashar al-Assad's troops as they battle rebels.
The latest attack comes hours after the Syrian army backed by Hezbollah fighters captured Yabrud, a former rebel bastion in Syria near the Lebanese border.
Immediately after news of the attack broke, a radical Sunni Islamist group claimed responsibility for the blast via Twitter.
"Liwa Ahrar al-Sunna in Baalbek officially takes responsibility for the heroic martyrdom (suicide) operation 'revenge for Yabrud' in the village of Al-Nabi Othman," the group said.
It also warned Hezbollah and the Lebanese army -- which radical Sunni groups in Lebanon say has sided with the Shiite movement: "Prepare for the transfer of the battle of Yabrud into Lebanese territory."
Lebanon is deeply divided over the war in neighbouring Syria. While Hezbollah and its allies back Assad, many of the country's Sunnis support the three-year revolt.