Survivors says Boko Haram extremists bombed out of forest camps are attacking remote northeast Nigerian villages, killing scores and kidnapping hundreds of women, children and young men.

Farmer Muhammad Ali says militants armed with guns and swords took away his wife and two daughters, aged 9 and 3, from Abbacheri village.

He is among 400 refugees who say the attacks started a week ago and they walked for six days to reach safety.

A police officer says at least four ailing and malnourished children have died among refugees in Jakana, a village 40 kilometers (25 miles) west of Maiduguri city.

Nigeria's air force has been bombarding camps in Sambisa Forest as soldiers launched a ground offensive.

Some 20,000 people have died in the 6-year-old Islamic uprising.