At least 20 people have been killed after unknown gunmen opened fire at a crowded bar near the Burundi capital, Bujumbura, officials say.The number of dead could rise as some people were critically wounded in the raid in the Gatumba area, they say.Burundi\'s last rebel group officially laid downs its arms in 2009 but sporadic attacks have continued.The BBC\'s Prime Ndikumagenge in Bujumbura says it is the most deadly attack since last year\'s disputed poll.Former rebel leader Agathon Rwasa withdrew from the presidential election and fled thecountry after his National Liberation Forces (FNL) accused the governing party of fraud.The government has blamed recent attacks on bandits but our correspondent says some fear a new rebel group has emerged.\"Dozens of people, some in [military] uniform and with Kalashnikov [rifles] and grenades entered \'Chez les Amis\' bar. They told everyone to lie down and began shooting,\" a survivor who lost two siblings and a friend told the AFP news agency.A doctor who only gave his name as Leonard said the hospital where he worked was \"totally overwhelmed\" by the number of wounded.\"We are lacking blood, equipment and medicine to treat all the injured,\" he said.AFP reports that dead bodies had been left in a parking lot at one hospital.Some 300,000 people are said to have been killed in Burundi\'s 12-year civil war between the minority Tutsi-dominated army and ethnic Hutu rebels.The conflict officially ended in 2005 with a peace deal which saw former rebel leader Pierre Nkurunziza was elected president but FNL rebels continued fighting.