Amnesty International has called for an immediate probe into the deaths of hundreds of prisoners in Nigerian detention facilities during a clampdown on anti-government fighters.

The international rights group announced on Monday that it had evidence, showing that numerous prisoners had been suffocated, starved and subjected to extra-judicial killings in the army-led operation in the northeast of Nigeria.

The report, released by the Rights group, said, a senior army officer has admitted that around 950 detainees, who died in custody during the first half of the year, were suspected of having links with Boko Haram militant group.

There have been several deadly clashes between the members of Boko Haram and the army in northeastern part of the African country, since Nigerian security forces intensified crackdown on the militant group, following President Goodluck Jonathan’s order in May.

The Nigerian forces have been raiding militant camps and launching air and ground attacks on suspected hideouts of the militants over the past two months.

Boko Haram says its aim is to topple the Nigerian government, accusing it of being pro-Western.

The group has also claimed responsibility for a number of deadly attacks in Nigeria since 2009.

Over 3,600 have been killed during the growing violence in the north of Africa’s most populous country over the past four years.

In May 2013, the Nigerian government imposed a state of emergency in three states in the northeast, saying Boko Haram had become a security threat in those states.