In fierce fighting today, Nigerian troops clashed with Islamic extremists who attacked Maiduguri, the biggest city in northeastern Nigeria. Dozens of combatants have been killed and wounded, soldiers and hospital workers said.

At the same time the insurgents continued scorched-earth attacks on villages some 200 kilometres  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.

The attacks come as US Secretary of State John Kerry arrived in Lagos, Nigeria's commercial capital nearly 1,500 km southwest of Maiduguri, for meetings with President Goodluck Jonathan and his chief rival over fears of violence around critical 14 February elections.

In Maiduguri, troops blocked roads into the city, inadvertently preventing civilians from escaping.

"Coordinated air and land operations are being conducted now" around Maiduguri, Defense Ministry spokesman Brig. Gen. Chris Olukolade said in a message posted on Twitter.

"We believe hundreds of thousands of civilians are now at grave risk," Amnesty International said.

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.