According to Lebanon’s state news agency, two grad rockets were launched at neighbourhoods in the south of the city, injuring four people.
The attack heighten fears that Hizbollah’s growing involvement in the Syrian civil war risks a sectarian backlash in Lebanon.
In a speech on Saturday, Hizbollah leader Sayyed Hassan Nasrallah said that his group was committed to fighting rebels in Syria, whom he depicted as extremists.
“We will continue to the end of the road, we accept this responsibility and will accept all sacrifices and expected consequences of this position,” said Mr Nasrallah, in his first remarks since a fierce battle for the strategic city of Qusair produced the Shia group’s biggest casualties yet in Syria.
“We will be the ones who bring it victory, God willing,” he added.
Hizbollah is part of a strategic anti-Israel alliance with Damascus and Tehran, but previously, Mr Nasrallah has portrayed Hizbollah’s actions in Syria, where president Bashar al Assad is battling a mainly Sunni rebellion, as motivated by a responsibility to protect Lebanese Shia living in the border areas.
In Saturday’s speech he portrayed a wider struggle, saying that Sunni extremists fighting on the rebel side in the border areas threatened not just local Shia but “all the Lebanese people.”
He also said that if Syria fell in to the hands of Sunni extremists, “Israel will enter Lebanon and impose its will.”
“Syria is the back of the resistance, and the resistance cannot stand, arms folded while its back is broken,” Mr Nasrallah told thousands of supporters via a video link from a secret location.
Hizbollah’s role in its neighbour’s civil war has been under scrutiny in the past week as its fighters have played a prominent role in the regime effort to retake Qusair from the rebels. One activist group estimates its casualties at more than 30.
Rebels fighting to topple President Bashar al-Assad told Reuters on Saturday that additional tanks and artillery had been deployed around opposition-held territory in Qusair, a Syrian town close to the Lebanese border.
“I’ve never seen a day like this since the battle started,” said Malek Ammar, an activist speaking from the town by Skype. “The shelling is so violent and heavy. It’s like they’re trying to destroy the city house by house.”
Rebels are largely surrounded in Qusair, a town of 30,000 that has become a strategic battleground. Assad’s forces want to take the area to secure a route between the capital Damascus and his stronghold on the Mediterranean coast, effectively dividing rebel-held territories in the north and south.