MAIDUGURI, Nigeria — In fierce fighting Sunday, Nigerian troops clashed with Islamic extremists who attacked Maiduguri, the biggest city in northeast Nigeria. Dozens of combatants have been killed and wounded, soldiers and hospital workers said.
"Coordinated air and land operations are being conducted now," Defense Ministry spokesman Brig. Gen. Chris Olukolade said in a message posted on Twitter.
Troops blocked roads leading into the city, which also is preventing civilians from escaping.
"We believe hundreds of thousands of civilians are now at grave risk," Amnesty International said.
The attacks come the day U.S. Secretary of State John Kerry is expected in Lagos, Nigeria's commercial capital nearly 1,000 miles (more than 1,500 kilometers) southwest of Maiduguri.
Aides said Kerry is concerned about possible violence around Nigeria's Feb. 14 elections and will meet with the main contenders, President Goodluck Jonathan and former military dictator Muhammadu Buhari.
Jonathan made a surprise visit to Maiduguri 10 days ago and pledged to crush the insurgents. But his repeated promises are ringing hollow as Boko Haram since August has seized and kept control of large swaths of the northeast, including key border crossings into Cameroon, Chad and Niger.
Boko Haram has denounced democracy and is fighting to impose its strict version of Shariah law across Nigeria, whose population of about 170 million is divided almost equally between Muslims in the north and Christians in the south.
The extremists attacked on three fronts around Maiduguri, the capital of Borno, soon after midnight and troops battled to hold them at bay, according to an officer involved in the fighting.
He said he saw dozens of combatants killed. A hospital worker said they are treating dozens of wounded soldiers. Both men spoke on condition of anonymity because they are not authorized to speak to the press.
Boko Haram on Jan. 3 seized a key military base and town on the border with Cameroon, killing hundreds of civilians and leaving the main road open to Maiduguri. The military said they were counter-attacking a week ago. But escaping civilians said there was no fighting and the insurgents retain control of the town of Baga.
Olukolade said the militants on Sunday simultaneously attacked Maiduguri and Monguno town, 180 kilometers (112 miles) away.
While soldiers were succeeding in holding off the attack on Maiduguri, it appeared they might lose Monguno, according to Abbas Gava, a spokesman for the civilian self-defense groups fighting Boko Haram.
Maiduguri is the birthplace of Boko Haram and has been attacked many times in the five-year Islamic insurgency that killed 10,000 people last year.