A huge explosion has rocked the centre of Gaza City, killing at least four people and injuring 20 others. It was not immediately clear what caused the blast that destroyed several properties in the eastern Shajaiyeh situated near the border with Israel. One of the houses is understood to belong to a well-known family of Hamas supporters, Palestinian police said. The Israeli air force has often targeted Hamas members in Gaza, but military sources denied Monday's blast. Tawfiq Abu Khoussa, a Palestinian interior ministry spokesman, called the blast "mysterious" and said an investigation was under way. Sometimes blasts in Gaza are caused by the accidental detonation of weapons caches kept by militant groups - and that certainly cannot be ruled out in this case, says the BBC's Alan Johnston in Gaza.