Civil Defence members try to extinguish the fire from burning buildings and cars at the site of an explosion in Beirut's southern suburbs, August 15, 2013. [Photo/Agencies]

More attacks threatened

"This is the second time that we decide the time and place of the battle ... And you will see more, God willing," the Brigades of Aisha statement said, describing Hezbollah and Nasrallah as Iranian agents.

"We send a message to our brothers in Lebanon, we ask you to stay away from all the Iranian colonies in Lebanon...because your blood is precious to us," a masked spokesman, flanked by two men brandishing rifles, said in the video.

"But Hassan Nasrallah is an agent of Iran and Israel and we promise him more and more (attacks)."

However, many Lebanese politicians blamed Israel. "The explosion was carefully prepared and one of the theories is that it could have been an Israeli retaliation for the Labouneh operation," Interior Minister Marwan Charbel said.

He was referring to an incident last week when four Israeli soldiers were wounded in southern Lebanon. Nasrallah said on Wednesday they were targeted by Hezbollah, which fought a month-long war with Israel in 2006.

Caretaker Prime Minister Najib Mikati declared Friday would be a day of mourning for the victims of the Beirut blast.  

There have been two previous attacks in southern Beirut this year, as Syria's conflagration seeps across the border. Two months before the July 9 car bomb, two rockets were fired into the area.

Sectarian violence fuelled by the Syrian conflict has also erupted in the Bekaa Valley and the Mediterranean port cities of Tripoli and Sidon, reflecting the renewed tensions spreading through the Middle East.

Lebanon's Sunni Muslims mostly support the rebels in Syria, while Shi'ites have largely supported Assad, who is part of the minority Alawite sect, an offshoot of Shi'ite Islam.

Nasrallah has promised that his group will continue fighting for Assad after it spearheaded the recapture of the strategic town of Qusair in June.

In October last year, a car bomb in the east of the capital killed a senior intelligence official, Wissam al-Hassan, who was close to the country's leading Sunni opposition party, which has supported the uprising in Syria.