Image copyright Reuters Image caption Yobe state has been under a state of emergency since May 2013 
 
 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. 
 
 Image copyright AFP Image caption Outdoor public viewing centres for watching football are popular across Nigeria 
 
 At the scene: Abdullahi Kaura Abubakar, Kaduna, northern Nigeria 
 
 Kaduna's football fans have mixed views. While some say they will now stay away from viewing centres because they are scared, others say they will still go, but with caution. 
 
 Many say they enjoy going to the centres too much to stop. As well as watching the games with their friends, they enjoy arguing, analyzing and betting on the matches. Others say that after the bombing, they will stay away as they do not want to lose their lives for the sake of watching a football game. 
 
 Lynda, a student and a football fan says: "During the Champions League final between Atletico and Real Madrid, I was in a viewing centre when I heard that another one was bombed in Jos. I was scared but I had to finishing watching the match before going home. 
 
 "In as much as they are bombing, we just hope that it does not get us, but it is not enough to make people stop going to viewing centres." 
 
 Image copyright AFP Image caption The north of Nigeria has been frequently hit by militant violence 
 
 There have also been warnings of potential attacks at venues showing the World Cup in East Africa - Somali Islamist group al-Shabab killed 76 people watching the 2010 World Cup final at two restaurants in the Ugandan capital, Kampala. 
 
 Both Boko Haram and al-Shabab say watching football is unIslamic - a view rejected by mainstream Muslims. 
 
 The Yobe 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. 
 
 'Daily attacks' 
 
 Media playback is unsupported on your device Media caption A 60-second guide to Boko Haram 
 
 Open-air viewing centres - where people pay to watch live football - are popular throughout Nigeria. 
 
 On 1 June at least 14 people were killed in a bomb attack on a bar in Adamawa that was screening a televised football match. No group claimed responsibility for the blast. 
 
 In March, many people were also killed in explosions while watching football in a video hall in Borno's Maiduguri town. On both occasions, Boko Haram was blamed for the blasts. 
 
 Boko Haram - which means "Western education is forbidden" in the Hausa language - wants to overthrow the government and create an Islamic state. 
 
 The group captured international attention in April after it abducted more than 200 girls in Chibok in Borno state. 
 
 The states of Borno, Yobe and Adamawa have been under a state of emergency since May 2013. Thousands have died in the north-east since Boko Haram launched an insurgency in 2009. 
 
 Nigeria's military has promised increased security but there are still daily reports of attacks on villages in parts of the north-east. ||||| DAMATURU, Nigeria (AP) — Witnesses say a suicide bomber in tricycle taxi triggered an explosion that blasted a World Cup viewing center in northeast Nigeria, killing several people. 
 
 Hospital workers say the death toll likely will rise with 15 people critically wounded and casualties still coming in at Damaturu, state capital of Yobe state. 
 
 Boko Haram Islamic extremists have targeted football viewing centers and sports bars in recent weeks, killing at least 40 people in two blasts in northern cities. 
 
 Witnesses said the tricycle taxi was driven into the outdoor area soon after the Brazil-Mexico match started Wednesday night. All spoke on condition of anonymity for fear of reprisals. 
 
 Nigeria's military has promised increased security but appears incapable of halting a stream of attacks by extremists holding more than 250 schoolgirls hostage.
– Boko Haram appears to have taken aim at World Cup soccer fans in Nigeria. A suicide bomber triggered a blast at an open-air viewing center in the northeast city of Damaturu, reports AFP. The AP has the death toll in the immediate aftermath at "several people," but it's expected to rise given the number of casualties being brought to local hospitals. Boko Haram has set off deadly bombs at two other sports-related venues in the last several weeks, prompting authorities in some locales to close their World Cup viewing centers, reports the BBC. Leader Abubakar Shekau views the sport as a distraction from religion.