A suicide bomber detonated a tricycle taxi packed with explosives at an outdoor World Cup viewing center in a northeast Nigerian city Tuesday night, and witnesses said several people were killed.

Hospital workers said the death likely will rise with 15 people critically wounded and casualties still coming in to the main hospital at Damaturu, capital of Yobe state.

Police Assistant Superintendent Nathan Cheghan confirmed the explosion but said rescue workers were being careful for fear of secondary explosions. Islamic extremists of the Boko Haram group frequently time secondary explosions to kill people who rush to the scene of a bomb blast.

Cheghan said he had no casualty figures.

There was no immediate claim for the blast witnesses were blaming on Boko Haram fighters who have targeted football viewing centers and sports bars in the past. Two explosions in recent weeks killed at least 40 people in two northern cities.

Boko Haram also claimed responsibility for the kidnapping of 234 female students in an incident that sparked international outrage this past April.

On Tuesday, witnesses to the bombing said the tricycle taxi was driven into the outdoor area soon after the Brazil-Mexico match started Tuesday night. All spoke on condition of anonymity for fear of reprisals.

One hospital worker said he saw at least seven bodies. Another said 15 casualties were in intensive care. Both asked that their names not be published because they are not authorized to speak to reporters.

Nigeria's military has promised increased security but appears incapable of halting a stream of attacks by the extremists, who are still holding the schoolgirls hostage.

The kidnapping of the girls two months ago and failure of Nigeria's military and government to rescue them has roused international concern. The United States is searching for the girls with drones and has sent experts along with Britain and France to help in counter-terrorism tactics and hostage negotiation.

Boko Haram's leader Abubakar Shekau has threatened to sell the girls into slavery unless the government agrees to exchange them for detained extremists, but President Goodluck Jonathan has said he will not exchange prisoners. Nigeria's military has said it knows where the girls are but that any military campaign could get them killed.

Boko Haram wants to enforce an Islamic state in Nigeria, Africa's biggest oil producer with a population almost equally divided between Christians and Muslims.

Nigeria has been under threat of terror attacks, and on Thursday, the state of Adamawa ordered all World Cup viewing sites to be closed, saying they had received credible intelligence of a terror attack. Both Yobe and Adamawa have been under a state of emergency since 2013.

A number of Islamic militant groups, including Boko Haram, have condemned the World Cup and the watching of soccer as "anti-Islamic."