BEIRUT — An al-Qaida-linked group claimed responsibility on Saturday for a suicide car bombing last week in a Shiite-dominated neighborhood in Lebanon, as its fighters fought other rebels in neighboring Syria in the most serious infighting since the uprising began.
The al-Qaida-linked Islamic State of Iraq and the Levant had not before claimed responsibility for an attack in Lebanon, underscoring how the ever-more-complex Syrian war is increasingly spilling over into its smaller neighbor.
The group may have rushed to claim responsibility to try to divert attention from the infighting in Syria, said Aymenn al-Tamimi, an expert on the country's militant groups.
At least five people were killed in the Thursday attack that targeted a south Beirut neighborhood that is bastion of support for the Shiite group Hezbollah.
ISIL vowed more attacks.
It was "the first small payment of a heavy account which these criminal hypocrites should wait for," it said in a statement, referring to Hezbollah. The statement was posted on a website used by Sunni militants.
The al-Qaida group sought to punish Hezbollah — and their backers among ordinary Lebanese Shiites — for sending fighters to Syria to shore up the forces of the Syrian president Bashar Assad, who is trying to quell an armed uprising.
In recent months, violence has targeted both Sunni and Shiite neighborhoods in Lebanon, further stoking sectarian tensions that already run high as each community lines up with its brethren in Syria on opposing sides of the war.
In Syria, meanwhile, hardline Sunnis dominating rebel groups have shown little tolerance for Syria's patchwork of minorities.
In response, Syrian minorities have rallied behind Assad or remained neutral, fearing for their future should Sunni extremists come to power.
ISIL is one of the strongest rebel groups in Syria.
It emerged from the Sunni heartland of neighboring Iraq, where it has also targeted Shiites with car bombs, sending the country to the brink of civil war.
Its fighters have fanned into Syria, taking advantage of the upheaval to assert power in areas seized by rebels. It has imposed its strict version of Islamic law, kidnapping and killing journalists, Syrian anti-Assad activists and others critical of their rule.
Tensions between ISIL and other rebel brigades have simmered for months. They erupted Friday after residents in rebel-held areas came out to demonstrate against the al-Qaida-linked group, accusing them of killing a man who was trying to mediate between the groups, said the U.K.-based al-Tamimi.
Fighting between ISIL and other anti-Assad groups spread quickly in some regions, said the Britain-based Syrian Observatory for Human Rights, saying it believed tens of fighters were killed.
Saturday evening, ISIL said in a video it would withdraw fighters from strategic strongholds that have defended rebel-held parts of the northern city of Aleppo against Assad forces if rebel attacks against it continued.