Lebanese security forces seized a car loaded with explosives and arrested four men suspected of preparing bombs, days after a deadly bombing in southern Beirut, security sources said on Sunday.

The car was discovered on Saturday about 15 kilometers (10 miles) south of the capital in Naameh, laden with five containers of TNT as well as nitroglycerin, they said.

The four men were being held on suspicion of preparing explosives for possible use in car bombings, but were not believed to be connected to Saturday's discovery or to the car bomb which killed 27 people three days ago.

That attack in a Shi'ite district of southern Beirut which is a stronghold of the Lebanese militant group Hezbollah was the deadliest in the capital since Lebanon's 1975-1990 civil war.

The attack, which Shi'ite Hezbollah blamed on radical Sunni Muslims, followed months of growing sectarian tension in Lebanon fuelled in part by Hezbollah's intervention against Sunni Muslim rebels in Syria's civil war.

The two-year conflict has killed 100,000 people inside Syria and the violence has spread across the Lebanese border, with rocket attacks in the Bekaa Valley, street fighting in the Mediterranean cities of Sidon and Tripoli and bombs in Beirut.

Security forces at checkpoints on the main coastal road to Beirut from southern Lebanon were stopping and searching vehicles on Sunday, drivers said.