MAIDUGURI, Nigeria — In fierce fighting Sunday that killed more than 200 combatants, 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 125 miles 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.

The multiple attacks come as U.S. Secretary of State John Kerry visited Lagos, Nigeria’s commercial capital nearly 1,000 miles southwest of Maiduguri, to encourage peaceful elections on Feb. 14.

“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.

Kerry promised more U.S. 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.