Abuja - A ringleader of the Islamist sect Boko Haram, who was believed to have masterminded last August's deadly attack on Nigeria's United Nations headquarters, has been captured and killed, authorities said on Thursday.

Habeeb Bama died of wounds suffered in a gun battle with riot police in the northern state of Yobe, authorities confirmed.

Bama's death comes after Boko Haram unleashed attacks on churches on Sunday in Kaduna, leaving at least 110 dead after reprisal attacks followed.

The US State Department said on Thursday that it would add three surviving leading figures in Boko Haram to its terrorist list, including Abubakar Shekau, whom the US called the sect's “most visible” leader.

“We took this measure to designate these three because they are clearly kingpins of Boko Haram, and clearly, all of them have advocated terrorism as a weapon in their campaign,” State Department spokesperson Victoria Nuland said in Washington. “They are all committed to violent extremism.”

She said the US is “continuing to look at the question of a broader designation” for Boko Haram but has yet to list the entire group as a terrorist organisation. Boko Haram is “loosely constructed”, and Washington is trying not to disrupt the Nigerian government's dialogue with northern groups to address their grievances, Nuland said.

Boko Haram, which is estimated to have slain at least 1 000 people in the last 18 months, has vowed to unleash fresh violence on Nigeria.

Security has been beefed up in Abuja as well as northern states such as Yobe, where a curfew has been in place since clashes between Christians and Muslims broke out Sunday. - Sapa-dpa