Boko Haram leader Abubakar Shekau directly ordered women to be killed in the northeast Nigerian town of Gwoza, one man who was forcibly conscripted into the militant ranks claimed on Friday.

Usman Ali said he witnessed the killing in the town, which the group's elusive leader proclaimed as part of a caliphate last year and which has generally been seen as the militants' headquarters.

Nigeria's military on Friday announced that troops had retaken the town of Gwoza from Boko Haram, from which the group declared their caliphate last year.

"Troops this morning captured Gwoza destroying the Headquarters of the Terrorists self-styled Caliphate," Defense Headquarters in Abuja said on Twitter.

The United Nations Human Rights Council will hold a special session next week on abuses by Boko Haram militants in Nigeria and neighboring countries, the council president said Friday.

The rights council rarely convenes special sessions, with only 22 held since the body was created in 2006.

The top U.S. diplomat for Africa will travel to Nigeria to observe presidential elections, the State Department said Thursday, an unusual move for Washington that is closely monitoring the country's Islamist crisis.

Assistant Secretary for African Affairs Linda Thomas-Greenfield will lead an "official diplomatic observation mission" of the presidential and national assembly elections that take place Saturday, according to a statement.

Nigeria's military has detained two Al-Jazeera television journalists in the country's northeast, where troops have been battling Boko Haram militants.

Defence spokesman Chris Olukolade said the pair were "found to have been loitering around areas where military operations are ongoing in the northeast and have been restrained in Maiduguri".

More than 1,000 civilians have been killed in Boko Haram attacks in the first three months of this year, Human Rights Watch said on Thursday.

"Each week that passes we learn of more brutal Boko Haram abuses against civilians," the group's Nigeria researcher, Mausi Segun, said in an emailed statement.

Mass abductions of children by groups like Boko Haram and the Islamic State are on the rise, with the practice now becoming a tactic of war, a U.N. envoy warned Wednesday.

Leila Zerrougui, the special representative for children and conflict, urged the Security Council to punish armed groups who target children with sanctions and strengthen measures to protect children in conflict.

Nigeria's government on Wednesday denied reports of a mass kidnapping in the country's northeast, as Boko Haram militants flee a four-nation military offensive.

"There is no fresh kidnapping in Damasak," Nigeria's national security spokesman Mike Omeri told Agence France-Presse, referring to the town recently retaken by forces from neighboring Chad and Niger.

Chad's U.N. envoy on Wednesday voiced frustration over Security Council inaction on endorsing a regional force fighting Boko Haram, arguing that the Nigerian extremists are "more dangerous" than Islamic State fighters.

Ambassador Mahamat Cherif told reporters that a draft resolution circulated last week to the 15-member council had hit a wall over a key provision invoking chapter 7 of the U.N. charter.

The head of Nigeria's army on Wednesday vowed a violent crackdown on election-related unrest, as security tightened before this weekend's presidential and parliamentary vote.

Lieutenant Colonel Kenneth Minimah said the armed forces, police and other organizations had made "adequate arrangements" for security at this Saturday's poll.