At least 15 separate explosions punctuated by gunfire echoed Monday across a northeast Nigeria city long under siege by the radical Islamist sect known as Boko Haram, witnesses said.

Attacks in Maiduguri, the sect's spiritual home, have grown increasingly commonplace, but the apparent sustained strikes, which began Monday afternoon and continued into the evening, saw soldiers lock down nearly every street in the city center. It remained unclear what was happening Monday night, as police and military officials did not answer repeated calls for comment as sirens wailed across the city. Witnesses at the scene of one of the blasts said a primary school was ablaze.

Workers and journalists found themselves trapped inside the Maiduguri office compound of the Borno state governor, as police officers manning the facility locked every gate and stood nervously on guard. Asked what was happening, one officer simply said: "The town is bad."

Earlier Monday, witnesses said a gunman suspected to belong to Boko Haram shot dead a police traffic warden in the city within view of a military checkpoint. The gunman simply walked away after the attack unstopped, witnesses said.

Boko Haram has been launching guerrilla attacks against Nigeria's weak central government for months as part of its efforts to free its imprisoned members and enact strict Islamic law across the multi-ethnic nation largely halved between Christians and Muslims. In recent days, it has begun a renewed offensive in Maiduguri.

On Oct. 5, a bombing by suspected Boko Haram members that killed an army lieutenant sparked a violent retaliation by soldiers. Troops opened fire with assault rifles and heavy machine guns mounted on armored personnel carriers on a busy street near the local headquarters of the Nigerian Union of Journalists, according to witnesses.

Afterward, an Associated Press journalist counted the bodies of more than 30 dead civilians and saw more than 50 shops and homes burned.

The military later denied it killed civilians, but offered contradictory explanations about what happened. Activists say they worry that other recent military strikes against Boko Haram may have killed civilians as well.

Boko Haram, which speaks to journalists in telephone conference calls at times of its choosing, could not be immediately reached for comment Monday night. The sect has killed more than 690 people this year alone, according to an AP count.