A suicide bomber killed two Lebanese soldiers and a civilian with a car bomb at an army checkpoint in a Hezbollah stronghold in northeast Lebanon on Saturday, security sources said.
The Nusra Front in Lebanon - a militant Sunni group named after one of the factions fighting against President Bashar al-Assad in Syria - said it carried out the attack, in a statement posted on Twitter and a website used by militant groups.
Hezbollah, an Iranian-backed political and military movement, is fighting alongside Assad's forces against predominantly Sunni Muslim rebels in a conflict that has exacerbated sectarian tensions in Lebanon.
The latest bombing was the third such attack in recent weeks in Hermel, a predominantly Shi'ite Muslim area near the border with Syria. Seventeen people were wounded.
Assad is a member of the Alawite sect, an offshoot of Shi'ite Islam.
The bombing followed a suicide attack in Beirut on Wednesday targeting the Iranian cultural center. A radical Sunni group, the Abdullah Azzam Brigades, said it had carried out that bombing, which killed eight people, as a reprisal for the military involvement of Hezbollah and Iran in the Syrian war.
The bomb was set off when soldiers at the checkpoint became suspicious of the man in the vehicle, a Jeep Grand Cherokee, the sources said.
Security is one of the main challenges facing a new Lebanese government that took office a week ago after the country went for nearly a year without a cabinet because of political tensions fuelled by the Syria conflict.
Prime Minister Tammam Salam, the highest-ranking Sunni Muslim in the administration, condemned Saturday's attack as an act of terrorism. He called for "solidarity in confronting terrorism in all its forms".
The security forces cordoned off the area after the blast.
Hezbollah is a long-standing ally of Damascus. It says it is fighting in Syria to prevent Sunni radicalism from spreading to Lebanon. Its Lebanese critics say it must pull out, saying Hezbollah's role in Syria is provoking such attacks.
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