A court has released 475 people allegedly affiliated with Boko Haram for rehabilitation, the justice ministry said on Sunday, as the country's biggest legal investigation of the militant Islamist insurgency continues.

The first person convicted for the kidnapping in 2014 of Chibok schoolgirls, sentenced to 15 years' imprisonment last week, was also handed an addition 15-year sentence, to run back-to-back, the justice ministry said in a statement.

More than 20,000 people have been killed and two million forced to flee their homes in northeastern Nigeria since Boko Haram began an insurgency in 2009 aimed at creating an Islamic state.

But humanitarian groups have criticised the Nigerian authorities' handling of those detained for infringing on the suspects' rights.

Some of those whose cases were heard last week in a detention centre in central Nigeria had been held without trial since 2010, according to the justice ministry statement.

"The prosecution counsel could not charge them (with) any offence due to lack of sufficient evidence against them," the ministry said.

In October, the ministry said 45 people suspected of Boko Haram links had been convicted and jailed. A further 468 suspects were discharged and 28 suspects were remanded for trial in Abuja or Minna.

Reuters