Thousands of Nigerian refugees who fled to southeast Niger to escape the Islamist group Boko Haram are in an "atrocious" situation, the United Nations says.

"There is a psychosis among the people who were expelled from their villages, their homes by Boko Haram," Toby Lanzer, UN regional humanitarian coordinator for the Sahel, told Agence France Presse Wednesday during a visit to Assaga, one of the largest Nigerian refugee camps in southeast Niger. 

Nigerian Islamist insurgent group Boko Haram has killed some 400 civilians since last year in northern Cameroon, with dozens more dying in a "heavy-handed" response by security forces, Amnesty International said in a report published Wednesday.

The report, entitled "Human rights under fire: attacks and violations in Cameroon's struggle with Boko Haram" and published in Yaounde, is based on three research missions in 2015, Amnesty said.

President Muhammadu Buhari told Agence France Presse on Wednesday that Nigerian authorities were talking to Boko Haram prisoners in their custody and could offer them amnesty if the extremist group hands over more than 200 schoolgirls abducted last year.

The Nigerian leader added that he was confident "conventional" attacks by the group would be rooted out by November -- but cautioned that deadly suicide attacks were likely to continue.

The trial of Ahmed al-Asir at the military court was on Tuesday postponed to October 20 upon the request of defense lawyers.

The military judge's decision to adjourn the session was expected because it was the first time that the Salafist cleric went on trial.

Six people died after a primary school collapsed at a town near Jos, central Nigeria, the National Emergency Management Agency (NEMA) said on Monday.

Three boys, two girls and a teacher were killed in the incident on Sunday at the Abu Naima school in Bukuru, NEMA local coordinator Mohammed Abdulsalam said.

President Muhammadu Buhari is eying closer ties with France, including on security, as Nigeria and its neighbors fight back against Boko Haram, his office said on Sunday.

The 72-year-old will spend three days in Paris from Monday following an invitation from his French counterpart Francois Hollande, Buhari's spokesman Femi Adesina said in an emailed statement.

At least seven people were killed and 20 others injured in a blast at a camp for people displaced by the Boko Haram conflict in northeast Nigeria, the country's main relief agency said on Wednesday.

A homemade bomb planted inside a tent went off shortly before 11:00 am (1000 GMT) at the Malkohi internally displaced persons (IDP) camp near the Adamawa state capital, Yola.

A military judge issued on Tuesday an arrest warrant against Ahmed al-Asir for forming sleeper cells and plotting the assassination of known figures, the state-run National News Agency reported.

NNA said Judge Najat Abu Shakra issued the warrant after questioning the Salafist sheikh.

Nigerian President Muhammadu Buhari will travel to Accra on Monday to discuss regional security with his Ghanaian counterpart, John Dramani Mahama, his office said in a statement.

During the one-day official visit Buhari, who on Saturday marked his first 100 days in office, will also discuss trade relations with Ghana, his office said.

Over 2.1 million people are internally displaced in northeast Nigeria after a spike in a six-year insurgency by the Boko Haram group, the International Organization for Migration said Friday, revising a previous figure of 1.5 million.

"The recent spike in attacks by insurgents triggered the increase," the IOM said, referring to stepped up attacks by the hardline Islamist group in recent months.