Nigeria World Cup viewers hit by deadly bomb blast

 A burnt car is seen along

 a road in Buni Gari village in Nigeria's northeastern state of Yobe, 6 April 2014 Yobe state has been under a state of emergency since May 2013 Continue reading the main story Boko Haram Relentless attacks Failed fight back Who are Boko Haram? In 60 secs: Boko Haram Watch At least 21 people have been killed in a bomb blast in northern Nigeria as they were watching a World Cup match, a hospital source has told the BBC. Witnesses in Damaturu, in Yobe state, say a suicide bomber in a tricycle taxi detonated explosives as people watched Brazil's match against Mexico on TV. At least 27 people are said to have been seriously injured. Public screenings of the World Cup in some parts of Nigeria have been banned because of threats by Boko Haram. Three states, including Yobe, are under a government-imposed state of emergency following years of attacks. No group has said it was behind the latest blast. Troops in northern Nigeria The north of Nigeria has been frequently hit by militant violence A man prepares barbecue popularly known as Suya for fans watching Nigeria's match against Ethiopia at a public viewing centre in Lagos - 29 January 2013 Outdoor public viewing centres for watching football are popular across Nigeria The local police chief has told the BBC that the attack happened outside a shop with a television, where people had gathered to watch the game. He said it was not a viewing centre where Nigerians often watch football matches, as these have been banned in the state. Other sources say it was a viewing centre which was attacked. While the police say 13 people died, the hospital worker told the BBC he had personally counted 21 bodies. Correspondents say Nigeria's authorities often downplay the number of casualties. In another development, the authorities say a senior Boko Haram militant was among almost 500 northerners arrested as they travelled to Nigeria's oil hub of Port Harcourt earlier this week. The authorities did not name the suspect. Relatives of those arrested say they had no links to Boko Haram and had gone to southern Nigeria for economic reasons. The Damaturu hospital worker said truckloads of injured people were being treated in overcrowded wards after the explosion on Tuesday evening. "The military and police trucks that brought them in have made four return trips so far ferrying them in," the worker said. "Every single truck was full of the injured. And all of them are young men or children." The worker said that the injuries suffered by people caught up in the blast were "horrific". Damaturu resident Mohammed Kurkure Yobe told the BBC that the venue where the attack took place was very popular and often crowded with people watching big events. 'Football un-Islamic' Open-air viewing centres - where people pay to watch live football - are popular throughout Nigeria. Boko Haram has described football as un-Islamic. Map of Nigeria On 1 June at least 14 people were killed in a bomb attack on a bar in