At least 38 people were wounded, a hospital official told Reuters.
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Reports that one employee had been killed in the parking lot where the explosion occurred in Beirut's southern suburbs could not be confirmed by hospital officials.
Hezbollah sealed off the site of the explosion, a shopping mall run by the group in the the Bir al-Abed district.
The officials said it was not clear whether the blast in the south Beirut suburb was caused by a car bomb. The officials, who spoke on condition of anonymity in line with regulations, said it was near a gas station.
Smoke rising in Beirut
Hezbollah's Al-Manar TV said the blast was a car bomb inside a parking lot near an Islamic center. The station broadcast footage of a thick plume of smoke rising into the sky at the site of the blast as people rushed to take casualties to the hospital.
"I haven't heard an explosion like this one since the 1980s(when a car bomb targeted Hezbollah's late spiritual leader Sayyed Mohammed Hussein Fadlallah)," a woman in southern Beirut said.
Interior Minister Marwan Charbel said the attack was "a criminal act aimed at destabilizing the country and creating Sunni-Shiite sectarian strife".
Despite internal tensions and the dispute with Syrian rebels, Hezbollah was quick to point a finger at Israel. Ali Amar, a Lebanese parliament member on behalf of Hezbollah said the explosion had clear "Israeli fingerprints." Another Hezbollah MK, Ali Meqdad, said: "This is the work of agents trying to create a rift in Lebanon."
Some Syrian rebel groups have threatened to strike in Lebanon after Hezbollah joined Syrian President Bashar Assad's troops in their battle against opposition fighters.
Beirut after blast
In May, two rockets slammed into a Hezbollah stronghold in south Beirut, wounding four people. The rockets struck hours after Hezbollah leader Sheikh Hassan Nasrallah vowed in a speech to help propel Assad to victory in Syria's civil war.
Hezbollah has openly joined the fight in Syria, and the group's fighters were instrumental in a recent regime victory when government forces regained control of the strategic town of Qusair near the Lebanese border.
AP and Reuters contributesd to this report
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