UN chief Ban Ki-moon on Friday condemned the deadly double car bombing in Lebanon and appealed for restraint in the fractured country split over the war in neighboring Syria.

"The secretary-general strongly condemns the two bomb explosions, shortly after Friday prayers, outside two mosques in the north Lebanese city of Tripoli," a UN statement said.

He went on to call on all Lebanese to "exercise restraint, to remain united, and to support their state institutions, particularly the security forces, in maintaining calm and order in Tripoli and throughout the country, and in preventing the recurrence of such destructive actions."

"The Secretary-General hopes that those responsible for such cowardly acts of violence will be brought to justice as soon as possible."

The two powerful car bombs killed 42 people and wounded hundreds in the deadliest attack since the country's 1975-1990 civil war.

The attack has further stoked fears that Syria's civil war could boil over into Lebanon, where clashes have periodically erupted between supporters and opponents of the regime in Damascus.