The United Nations' human rights chief expressed concern over "persistent and credible" reports of human rights violations by the multinational task force created to fight the Islamist group Boko Haram.

After a sustained six-year insurgency in Nigeria's northeastern regions, Islamist group Boko Haram has attempted to expand its terror activities to border towns in Cameroon.

The expansion prompted the Lake Chad Basin nations Nigeria, Niger, Chad and Cameroon to form a multinational task force to combat the extremist group. The four nations are joined by Nigeria's western neighbour Benin.

The task force of around 8,000 military personnel is endorsed by the African Union and is currently seeking approval from the UN. It has also steadily gained ground and recaptured several towns in northeastern Nigeria from the militants.

However, UN's human rights official Zeid Raad al-Hussein told members of the UN Human Rights Council that his office had received reports of "arbitrary arrests" of ethnic Kanuris by military personnel belonging to the multinational task force, according to the National Post.

Mohammed Yusuf, who originally founded Boko Haram in 2009, is from the Kanuri tribe of northeastern Nigeria.

"Kanuris are now considered suspect by some military personnel," Zeid told the Council during the special session in Geneva.

Zeid also revealed during the session that the Boko Haram is now targeting Nigerians with Shuwa Arab ancestry "in retaliation for their perceived support to the Nigerian armed forces."

"There is thus a high risk of escalating ethnic and religious violence," Zeid warned the council. "This can only be halted by principled leadership and clear instructions to military personnel, with appropriate accountability."

The human rights official expressed concern "about the growing ethnic and sectarian dimensions of the conflict," and called for "thorough and fully transparent investigations" by authorities governing the multinational task force currently fighting the Islamist group.