Five people have been killed in an attack on the main law courts in the Somali capital, police have said. Gunmen entered the Mogadishu courts, detonating explosives and opening fire, witnesses said. A gunfight followed as security forces exchanged fire with the attackers, who remain unidentified. Most attacks in Mogadishu are blamed on Islamic militant group al-Shabab. Somalia has been rebuilding itself after two decades of civil conflict. A BBC reporter says armed intruders entered the court in Mogadishu and began firing, after which there was an explosion. An intelligence officer, Abdirahman Mohamed, told the French news agency AFP that at least one suicide bomber had blown himself up, leaving several people dead. "Armed men entered the court and then we heard a blast. Then they started opening fire," witness Hussein Ali, who works at the courts, told Reuters news agency. Other witnesses told AFP the attackers were dressed in Somali military uniform. The incident comes a month after 10 people were killed in a suicide car bombing, in one of the deadliest attacks in the coastal city since a new UN-backed Somali government was formed last year Security has improved in Mogadishu since al-Shabab withdrew from the city in August 2011, but the group's fighters still launch attacks. The al-Qaeda-aligned insurgents have been forced out of main towns but still control most villages and rural areas. For more than 20 years Somalia has seen clan-based warlords, rival politicians and Islamist militants battling for control of the country.