By Felix Onuah

ABUJA (Reuters) - Nigeria's President Muhammadu Buhari on Wednesday told a group of 21 girls released by Islamist militants last week that he will "redouble" efforts to bring home about 200 still missing after the kidnapping in Chibok in 2014.

About 270 girls were seized at their school in northeastern Borno state, where Boko Haram has waged a seven-year insurgency, killing thousands of people and displacing more than 2 million.

The kidnapping prompted outrage worldwide and the girls' plight was publicized with Twitter hashtag #bringbackourgirls.

"We shall redouble our efforts to bring the rest back home," Buhari said. "Aside from rescuing them, we are assuming the responsibility for their personal, educational and professional goals and ambitions in life.

"It is not late for the girls to go back to school and continue the pursuit of their studies," he said.

Boko Haram controlled a swathe of land about the size of Belgium at the start of 2015, but Nigeria's army, aided by troops from neighboring countries, has recaptured most of it.

The group still stages suicide bombings in the northeast as well as in Niger and Cameroon.

(Reporting by Felix Onuah; Writing by Ulf Laessing; Editing by Louise Ireland)