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

Hospital workers said the death toll 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 Boko Haram fighters, who have targeted sports viewing centers and bars in the past. Two explosions in recent weeks killed at least 40 people in two 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.

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 Both asked that their names not be published because they are not authorized to speak to reporters.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 Boko Haram, which is holding hundreds of schoolgirls hostage.

Boko Haram, which emerged more than a decade ago, wants to establish an Islamic state throughout Nigeria, and bitterly opposes Western education and culture. Its attacks have become increasingly violent, frequently targeting civilians instead of military and police.

In recent months, extremists have attacked schools, slaughtering students and teachers, detonated bombs in crowded markets and bus stations, and attacked towns and villages.

As the World Cup began last week, security experts warned that Islamic militants might attack crowds watching the games in public places, as they did in 2010 in Uganda.

World Cup screening venues in 10 countries — Nigeria, Somalia, Kenya, Iraq, Tanzania, Uganda, Djibouti, Burundi, Ethiopia, Tunisia — face risk of attack, according to Robert Besseling, the lead Africa analyst at the consulting firm IHS Country Risk. He wrote that there would “almost certainly” be attacks in Nigeria and that “the most likely locations where foreigners will be targeted are Kenya and Tanzania.”

The Kyadondo Rugby Club in Kampala, Uganda, was one of two spots attacked by al-Shabab militants during the 2010 World Cup final. The blasts killed 74 people.

The U.S. Embassy in Kenya this month warned citizens to “exercise caution” at venues with World Cup crowds. The British government warned that in Nigeria “terrorists have previously targeted places where football matches are being viewed.” Ugandan police and the U.S. Embassy in Kampala have issued alerts about impending attacks.

“One is quite on edge. And we don’t anticipate bad news but it’s lingering,” said Philippe Huenermann, the owner of Havana, a popular bar in Nairobi, Kenya. “It’s like a shadow.”