Story highlights The Biafra movement calls for an independent state in the majority Igbo region of Nigeria

Amnesty International says many abuses occurred around Biafra Remembrance Day celebrations

(CNN) Nigerian security forces have killed at least 150 people in a series of attacks against the largely peaceful pro-Biafra secessionist movement, according to a report by Amnesty International.

The report finds "overwhelming evidence that Nigerian security forces committed gross human rights violations" at a number of public gatherings in the country's southern region since August 2015.

Pro-Biafran protestors gather in Onitsha, Nigeria on May 30 2016.

In what the human rights group calls "a chilling campaign," security forces were apparently responsible for the extrajudicial killings, detentions, and torture of supporters of the Indigenous People of Biafra (IPOB), a group that is pushing for an independent Biafran state in Nigeria's oil-rich southeast.

"This deadly repression of pro-Biafra activists is further stoking tensions in the southeast of Nigeria," said Makmid Kamara, Interim Director of Amnesty International Nigeria

"This reckless and trigger-happy approach to crowd control has caused at least 150 deaths and we fear the actual total might be far higher," Kamara said.

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