New York - United Nations envoy Gordon Brown urged Boko Haram on Friday to free the more than 200 schoolgirls it has held for 10 months.
His appeal came after the reported release this week of women and girls kidnapped by the Islamist group in December.
"Now they have released some hostages, they should release them all," said Brown, a former British prime minister, in a statement.
He referred to 158 women and girls who were said to have been released by the militants on Thursday after being taken during a raid on the village of Katarko.
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The abduction of scores of schoolgirls from a government secondary school in the town of Chibok on April 14, 2014 sparked global outrage and offers of international assistance and a worldwide social media campaign with the Twitter hashtag #BringBackOurGirls.
But the location of the schoolgirls, most of whom are believed to be between the ages of 16 and 18, remains unknown.
"Boko Haram are piling cruelty upon cruelty by failing to free the girls," said Brown, who is the U.N. Special Envoy for Global Education.
He said there would be no let up in the campaign to find and free the girls. If they are not released by the first anniversary of their captivity, a vigil will be held at the United Nations in New York on April 14.