TRIPOLI  - At least one person was killed and 20 were wounded on Saturday as a powerful blast shook a six-storey residential building in the northern city of Tripoli, a Lebanese security official said. "One man was killed and some 20 people, most of them women and children, were wounded," the official told AFP as rescuers evacuated victims from the site of the 5:30 am (0230 GMT) blast in the Sunni area of Bab al-Tebbaneh. It was not known several hours later what caused the explosion in a district of the tense northern city that has been at the centre of clashes between Sunni militants and fighters of the Alawite community, an offshoot of Shiite Islam. An official at Tripoli's Islamic Charity Hospital said 17 of those wounded were given first aid for light injuries and released while three others were kept in for treatment. Panicked residents, some still in their pyjamas, were seen fleeing the area that has been the scene of fierce sectarian clashes. The first floor of the building, with four apartments, was destroyed by the explosion along with several stores on the ground floor. Cars parked nearby were damaged. Lebanese troops and police reinforcements were deployed in the area. Residents of Bab al-Tebbaneh who support the Western-backed majority in parliament have clashed repeatedly with Alawites in the nearby Jabal Mohsen district who back the Hezbollah-led opposition supported by Syria and Iran. Nine people were killed and some 45 wounded in clashes between the two sides on June 22 and 23. On Friday night, two men were wounded when a grenade went off, an army spokesman, adding that one of the men had his leg torn off by the blast, which may have been accidental. "Based on our initial investigation, the man who lost his leg was probably holding the grenade when it fell and exploded," the spokesman told AFP. The Tripoli clashes have raised fears of a nationwide security breakdown amid stalled efforts by Prime Minister Fuad Siniora to form a national unity cabinet since a Qatari-brokered deal in May to end an 18-month political crisis. The accord between the opposition and ruling coalition led to the election of army chief Michel Sleiman as president, ending a six-month vacuum in the top job. But rival factions have continued to bicker over the distribution of key portfolios in the new 30-member government. The deal was struck after at least 65 people were killed in May in sectarian clashes that saw Hezbollah stage a spectacular takeover of mainly Sunni areas of west Beirut. In the Tripoli violence, about 30 buildings and 15 stores were burnt and 57 buildings damaged by rocket and gunfire. Combatants traded heavy machine-gun fire, mortar rounds and rocket-propelled grenades in the densely-populated Tripoli neighbourhoods of Bab al-Tebbaneh, Jabal Mohsen and Al-Qobbe. But on Wednesday, fighters dismantled barricades in areas under their control and the army started street patrols.