A car bomb ripped through a Shi'ite-dominated area of Lebanon's capital Tuesday, killing at least four people and wounding 35.
A Lebanese security official says a suicide bomber drove a stolen car packed with explosives into the blast site.
A Sunni extremist group known as the Nusra Front in Lebanon claimed responsibility, saying on Twitter the attack was in retaliation for Hezbollah's military support of Syrian President Bashar al-Assad.
The blast occurred in southern Beirut, a Hezbollah stronghold. A car bomb killed five people in the same area earlier this month.
State Department spokeswoman Marie Harf says the United States condemns the attack in the strongest terms. She says such abhorrent acts of terrorism undermine the peace and security the Lebanese people deserve.
Hezbollah fighters have openly battled in neighboring Syria since last year, helping Mr. Assad's forces against Sunni rebels trying to oust him.
Tensions have increased between Lebanon's Shi'ite and Sunni communities as armed clashes and bombings spread into Lebanon.