By HARUNA UMAR Associated Press

A bomb exploded Saturday afternoon near Maiduguri's main market, after the city was attacked before dawn by Islamic extremists by rocket-propelled grenades that killed at least 13 people, said residents.

The explosion was by the mosque near the marketplace of Maiduguri, the largest city in Nigeria's troubled northeast.

The early morning grenade attack was a new tactic by Boko Haram in the extremist group's birthplace.

Maiduguri resident Mari Madu said he counted at least 40 thunderous blasts from the grenades that began around 1 a.m. before he lost count.

"Each time they fired into the town, we saw bright sparkling flashes which moved with great speed ... One of the blasts shook my roof so badly that I thought it must have landed on my house," he said.

Several homes were destroyed in Maiduguri's Dala-Lawanti suburb, about 20 kilometers west of the city center, he said.

At least 13 people were killed and more injured by the grenade attacks, said Maiduguri resident Idrissa Mandara.

"Eight persons were killed in my compound. Five died instantly and three died at the hospital. We also have five others that have been buried after they were found dead in their homes," ?said Mandara. Among the dead were five children, two women and six men, he said.

An intelligence officer said Boko Haram was firing rocket-propelled grenades. Soldiers fired heavy assault rifles in response, said the officer, who insisted on anonymity because he is not supposed to give information to journalists.

A group of civilian fighters patrolled until dawn to ensure the Islamic extremists did not get through barriers of sandbags and trenches, said one of the fighters, Abbas Gava.

Maiduguri's population of 2 million has swelled with hundreds of thousands of refugees. More than 1.5 million Nigerians have been forced from their homes by the nearly 6-year-old Islamic uprising in the northeast which has killed an estimated 13,000 people.

A multinational offensive this year forced the insurgents from towns where they had declared an Islamic caliphate but the extremists continue to carry out suicide bombings and hit-and-run attacks.

In a separate incident, twin explosions at a village wedding venue killed seven people on Friday.

Nigeria's military says Boko Haram is trapped in the northeastern Sambisa Forest. But the insurgents keep attacking Maiduguri which is 200 kilometers (125 miles) away.

At his inauguration Friday, Nigeria's new President Muhammadu Buhari vowed to crush the extremists. Buhari said he will move the headquarters for the war against Boko Haram from Abuja, the capital in central Nigeria, to Maiduguri.

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Associated Press writer Michelle Faul contributed to this report from Abuja, Nigeria.