Boko Haram militants dressed as soldiers slaughtered at least 200 civilians in three communities in northeastern Nigeria, in massacres the military did nothing to stop despite warnings that an attack was imminent, witnesses said on Thursday.

A community leader who witnessed the killings on Monday said residents of the Gwoza local government district in Borno state had pleaded for the military to send soldiers to protect the area after they heard that militants were about to attack, but help didn't arrive.

It took a few days for survivors to get word of the massacres to Maiduguri, the provincial capital, because travel on the roads is extremely dangerous and phone connections are poor or nonexistent.

The slaughter was confirmed by Mohammed Ali Ndume, a senator representing Borno and whose hometown is Gwoza, and by a top security official in Maiduguri who insisted on anonymity because he isn't allowed to speak to the media.

Militants of Boko Haram, which wants to establish an Islamic state in Nigeria, have been taking over villages in the northeast, killing and terrorizing civilians and political leaders. The Islamist fighters are making a comeback from a year-long military offensive aimed at crushing them.

Thousands of people have been killed in the five-year-old insurgency - more than 2,000 so far this year - and an estimated 750,000 Nigerians have been driven from their homes.

Nigeria's military has insisted that it has the extremists on the run, making use of a big influx of troops and a year-old state of emergency in three states, which gives it the power to detain suspects, take over buildings and lock down any area.

But while Boko Haram has in large part been pushed out of cities in the northeast, it has been seizing villages with thatched-roof huts in the semi-arid region where it once held sway, boldly staking its claim by hoisting black flags with white Arabic lettering, and making large areas of Nigeria no-go regions for the military.

The villages attacked on Monday are near Gwoza, a regional political center whose emir was killed in a Boko Haram ambush on his convoy last week. Emirs are religious and traditional rulers who have been targeted for speaking out against Boko Haram's extremism.

Borno Governor Kashim Shettima traveled on Saturday to Gwoza to pay his respects to the fallen emir and was quoted by the Information Nigeria website as saying it was a terrifying ride.

A local journalist who was in the convoy, which was escorted by 150 soldiers, counted at least 16 towns and villages that were deserted along the 135-km route, according to a local media report.