Hundreds of people were killed in raids by Boko Haram Islamic militants in northeast Nigeria's Borno state, on the border with Cameroon, with some sources putting the death toll at 400 to 500.

On Tuesday, heavily armed men dressed as soldiers in all-terrain vehicles and on motorcycles attacked neighboring Goshe, Attagara, Agapalwa and Aganjara villages in Gwoza district, shooting residents to death and burning homes.

The attacks forced surviving villagers to flee to Cameroon and into the Mandara Mountains on the border.

"The killings are massive. Nobody can say how many people were killed, but the figure runs into some hundreds," said Peter Biye, a lawmaker in Nigeria's lower parliament representing the Gwoza region.

"The area is still under the control of the insurgents, and residents can't go back to bury the dead because of the danger involved," he said.

On Wednesday, a military jet bombarded Boko Haram positions to dislodge the militants from the villages they have occupied, forcing them to temporarily withdraw.

"They returned immediately after the jet left, making it too risky for villagers to return to bury the dead," Biye said.

Dead bodies litter the area around the attacked villages. Ground troops have yet to go to the area to push out the insurgents, he said.

The attackers, who posed as soldiers, told residents they had come to protect them from Boko Haram and asked them to assemble. They singled out men and boys and opened fire on them, Biye said.

A local leader in Attagara village, who fled to nearby Madagali town in neighboring Adamawa state, said the death toll was staggering.

"The death is unimaginable. We have lost between 400 and 500 people in the attacks in which men and male children were not spared," said the local leader, who asked not to be named for security reasons.

"The gunmen pursued on motorcycle people who fled into the bush in a bid to escape and shot them dead.

"Even nursing mothers had their male infants snatched from their backs and shot dead before their eyes," the local leader said.

The insurgents destroyed mobile phone towers in the region, so news of attacks is usually slow to emerge and verification of death tolls difficult to obtain.

If the death toll is confirmed, it will be the worst attack by Boko Haram in its five-year insurgency that has killed thousands, mostly in the northeast.

On May 5, the group killed 315 people in the town of Gamboru Ngala on the border with Cameroon not far from Gwoza, where they burned a market, businesses and several homes.

Meanwhile, dozens of Boko Haram fighters stormed Madagali town early Thursday, burning a church and a local government administrative building after subduing military and police personnel, a local official said.

Residents said two civilians were killed in crossfire during a shootout between soldiers and the gunmen.

"They came around 6 a.m. ... in 10 all-terrain vehicles and on several motorcycles, all dressed in military uniforms," said Maina Ularamu, a Madagali local government chairman.

The gunmen overran a police checkpoint, forcing policemen to flee, and then opened fire on a military checkpoint just outside the town, where they engaged soldiers in a shootout.

After overwhelming the soldiers who withdrew from the checkpoint, the gunmen attacked and burned a Roman Catholic church and a local government office opposite it, Ularamu said.