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 hadpay 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