Maiduguri - In fierce fighting that killed more than 200 combatants on Sunday, Nigerian troops clashed with Islamic extremists who attacked Maiduguri, the biggest city in northeastern Nigeria, from three fronts.At the same time the insurgents continued scorched-earth attacks on villages some 200km to the south in Adamawa state, slitting throats of residents, looting and burning homes and abducting dozens of trapped women and children, according to Vandu Kainu and other escaping survivors.Adamawa state legislator Adamu Kamale appealed for troops to protect civilians in Michika, where six villages are under attack. "The attacks have continued since Friday with no presence of security operatives," he complained.John Kerry visitThe multiple attacks come as US Secretary of State John Kerry visited Lagos, Nigeria's commercial capital more than 1 500km southwest of Maiduguri, to encourage peaceful elections on 14 February in Africa's most populous country."This will be the largest democratic election on the continent," Kerry said. "Given the stakes, it's absolutely critical that these elections be conducted peacefully - that they are credible, transparent and accountable."Kerry met with President Goodluck Jonathan and his chief rival candidate, former military dictator Muhammadu Buhari. Kerry told reporters afterward that he won pledges from both to refrain from violence.He also issued a warning: Anyone responsible for inciting post-election mayhem will be barred entry to the United States, where millions of Nigerians live.Fight against Boko HaramKerry promised more US support in the fight against Boko Haram if the elections take place peacefully and democratically.More than 800 people were killed in northern protests after Buhari, a Muslim northerner, lost 2011 elections to Jonathan, a Christian from the south.Boko Haram has denounced democracy and wants to make an Islamic state of Nigeria, whose population of about 170 million is divided almost equally between Muslims in the north and Christians in the south.In Maiduguri, troops blocked roads into the city, which also prevented civilians from escaping.Defence ministry spokesperson Brigadier General Chris Olukolade said on Sunday evening that troops successfully repelled attacks on Maiduguri and Konduga, 40km to the southeast.But he said they were mounting air raids in Monduno, a town 140km northeast of Maiduguri, which Boko Haram seized on Sunday morning.More than 200 combatants died on Sunday, mainly insurgents, according to soldiers and civilian self-defence fighters who counted bodies. They spoke on condition of anonymity because they are not allowed to speak to reporters.President 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.Multinational forceWith encouragement from the United Nations, Nigeria and its neighbours are setting up a multinational force to fight the extremists who recently have increased cross-border raids into Cameroon.But there is distrust of Nigeria's military, which many believe is infiltrated by Boko Haram at the highest levels.The Maiduguri attack is not unexpected. Boko Haram on 3 January seized a key military base and Baga 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 their control.Maiduguri is the birthplace of Boko Haram and has been attacked many times in the 5-year Islamic insurgency that killed at least 10 000 people last year.