At least eight people have been killed after female suicide bombers attacked two camps hosting internally displaced people (IDP) in Maiduguri, northeastern Nigeria, a civilian self-defence group said.

It was the first major attack on a displaced persons camp in the city which is the birthplace of Boko Haram.

The attack started late Sunday night and left another 15 people wounded, the Civilian-JTF group spokesman Bello Danbatta told The Associated Press.

Boko Haram often targets the city with suicide bombers and has been using female bombers increasingly.

Late last year, Nigeria's government declared the group "crushed" but dozens of such attacks have taken place in 2017.

The latest bombings occurred a few days after Nigeria's army chief of staff issued a 40-day deadline for troops to flush out Boko Haram's leader and finish off the group.

Danbatta said one bomber sneaked into the Dalori camp and detonated, and two other attackers exploded on or near the camp's perimeter fence. Another bomber detonated early on Monday.

Thousands of people continue to shelter in camps after being forced from their homes by Boko Haram.

Attacks carried out by the group over the last eight years have killed more than 20,000 people, kidnapped thousands of others, spilled into neighbouring countries and created one of the world's largest humanitarian crises.

Nigeria is moving closer to famine, with more than five million people expected to face "crisis, emergency and famine conditions" by the end of August as the lean season continues, the Norwegian Refugee Council said in a statement on Monday.