A suicide car bomber killed four people at a petrol station in a stronghold of the Shi'ite militant Hezbollah movement on Lebanon's northern border on Saturday, the latest sign that Syria's civil war is spilling over into its small neighbour. 

The blast occurred in the town of Hermel at the northern end of the Bekaa Valley, an area populated mainly by Shi'ite Muslims among whom Hezbollah draws its support.

Lebanon's National News Agency cited witnesses who said the perpetrator entered the gas station and asked to buy fuel before detonating the bomb, leaving a meter-deep hole in the ground and setting the station and nearby cars on fire.

A security source told Reuters that, besides the three dead bystanders and the dead bomber, 28 other people had been wounded in the blast.

There was no immediate claim of responsibility but Saturday's blast fitted a pattern of attacks by rival sectarian groups on each other's strongholds that has been amplified by Syria's civil war. Another suicide car bomb killed three people in Hermel last month.

Hezbollah has sent fighters and advisers to aid President Bashar al-Assad, a member of Syria's Alawite minority, which is an offshoot of Shi'ite Islam. Both Hezbollah and Assad are supported by Shi'ite-ruled Iran.

Hezbollah's intervention in Syria and the steady flow of Lebanese Sunnis joining the anti-Assad rebels have both fuelled sectarian strife in Lebanon, which has taken in more than 900,000 refugees from the Syrian civil war.

The spokesperson for the UN Secretary General Ban Ki Moon released a statement condemning the attack and extended his condolences to the families of the victims. 

"The recent escalation in acts of terrorism and violence in Lebanonis of grave concern," read the statement. "The Secretary-General reiterates his call on all Lebanese to rally behind their state institutions, including the army and security forces, as the best means to confront such unacceptable and indiscriminate actions and to safeguard their country's security and stability."