A suicide car bomb attack killed four people late Sunday in an area dominated by the powerful Shiite movement Hezbollah near the Syrian border, a Lebanese security source told AFP.
The attack in the Bekaa Valley was claimed by the jihadist Al-Nusra Front in Lebanon, as well as by a little known extremist Sunni Muslim group based in the region.
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.
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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.
After the attack, Al-Nusra Front in Lebanon claimed responsibility via Twitter, describing it as a "quick response to the bravado... of the party of Iran (Hezbollah) for their rape of Yabrud."
The group, with suspected links to Syria's Al-Qaeda affiliate Al-Nusra Front, also warned Hezbollah of "jihadist operations to silence their tongues from singing about this shameful rape" or takeover of Yabrud.
Earlier, the extremist Liwa Ahrar al-Sunna in Baalbek had also taken responsibility for the attack, describing it as "revenge for Yabrud."
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.