At least 12 people have been killed in a night of violence across the Burundian capital of Bujumbura after a government deadline to turn in illegal weapons expired.According to medical and official sources, a group of assailants targeted a bar Saturday night in Muha district, killing at least eight people.A local district head, Jean Bosco Girukwishaka, told Anadolu Agency that the five attackers were immediately detained by the police.The assailants' vehicle was also found Sunday morning about 500 meters away from the scene by the police, Girukwishaka added.According to medical sources, several injured victims were also taken to the hospital.Separately, four bodies were found by Burundian police in Bujumbura's northern district of Ntahangwa, according to a local district head.Remy Barampama said that the circumstances of the four deaths were still unknown but confirmed that an investigation was underway.On Monday, President Pierre Nkurunziza declared a five-day ultimatum on illegal weapons and warned of strict police action if civilians did not surrender such firearms.The latest shooting incidents occurred after Nkurunziza's ultimatum expired Saturday.He warned opposition groups to lay down their arms and surrender to the authorities in exchange for an amnesty.This has been an attempt to restore calm in the country where violence over the past six months has left more than 200 people dead.On Thursday, hundreds of families in Bujumbura started to flee their homes, anticipating an outbreak of violence.The African country has been gripped by instability since April when the ruling party named Nkurunziza -- in power since 2005 -- as its candidate for the presidency.UN Secretary-General Ban Ki-moon voiced alarm Friday about the ongoing escalation in Burundi, while condemning public statements aimed at inciting violence and hatred.According to a local NGO, at least 200 people, mostly opposition politicians but also Nkurunziza supporters, have been killed amid political turbulence that began when the president announced plans to run for a third term.Burundi's current army was drawn up from former rebel fighters after the 2005 Arusha peace accords ended a years-long civil war that left more than 300,000 people dead.