An Al Qaeda linked group claimed responsibility on Saturday for a suicide car bombing last week in a Shiite-dominated neighborhood in Lebanon, as its fighters clashed with other rebels in neighboring Syria.
The Islamic State of Iraq and the Levant's operations, now in both countries, underscore how the ever more complex Syrian war is increasingly spilling over into its smaller neighbor.
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 claim was posted on a website used by Sunni militants. The U.S.-based SITE Intelligence Group, which tracks militant websites, also published the news.
The Al Qaeda group sought to punish Hezbollah -- and their ordinary Shiite Lebanese backers -- for sending fighters to Syria.
It was the latest in a wave of attacks to hit Lebanon in recent months. The violence has targeted both Sunni and Shiite neighborhoods, further stoking sectarian tensions that are already running high as each community in Lebanon lines up with its brethren in Syria on opposing sides of the war.
But the bombing also reflected how Lebanese are turning on each other. Also Saturday, Lebanese authorities confirmed the identity of the suicide bomber, the state news agency reported. Local media identified him as a citizen from northern Lebanon.
Thursday's bombing came a week after a car bombing in downtown Beirut killed prominent Sunni politician Mohammed Chatah. The former finance minister and top aide to ex-Prime Minister Saad Hariri was critical of Syrian President Bashar Assad and his Hezbollah allies.
In November, suicide bombers targeted the Iranian Embassy in Beirut, killing at least 23 people. Iran is the chief patron of Hezbollah and an ally of Syria, and the Islamic Republic's embassy is located in an upscale Shiite district.
Another blast in August killed around 20 people in the Beir al-Abed district, near the Haret Hreik neighborhood where Thursday's bombing took place.
Two weeks later, a double bombing outside two Sunni mosques in the northern city of Tripoli killed scores more.
The tensions in Lebanon reflect the increasingly sectarian nature of the Syrian war, where hardline Sunni rebels 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, assaulted, kidnapped and killed journalists, Syrian anti-Assad activists and others it sees as critics.
But in recent days, tensions between Islamic rebels and ISIL have boiled over, after fighters of the Al Qaeda group were accused of killing a popular doctor.
On Saturday, they gave their Al Qaeda-linked rivals 24 hours to surrender.
The Britain-based Syrian Observatory for Human Rights said rebels from an alliance of Islamic groups attacked positions of the fighters from the militant Islamic State of Iraq and the Levant, killing and capturing dozens in fighting that has spread through the northern provinces of Idlib and Aleppo.
Ahmad al-Khatib, an activist in the Jabal al-Zawiya region in northwestern Idlib, reported the ultimatum.
The Western-backed coalition of Syrian opposition groups in exile welcomed the attacks on ISIL, calling the Al Qaeda fighters "an extension of the Assad regime."
In a statement, the Syrian Coalition said it called on the international community to support moderate fighters against the Assad regime.