America's U.N. ambassador says the U.S. will never give up trying to bring home the Chibok (chih-BAWK') girls.

Two years after Boko Haram abducted the girls from their school, Samantha Power visited with those holding vigil each day in Nigeria's capital.

A total of 219 remain missing, and Power says she can't imagine the frustration of theier families.

Power says the U.S. is working with Nigeria and neighboring countries to defeat Boko Haram and safely recover them and thousands more who've been kidnapped, too.

But she acknowledges that even a superpower can't snap its fingers and make that possible.

In Abuja, Power also met President Muhammadu Buhari (moo-HAH'-mah-doo boo-HAH'-ree) and other Nigerian officials. She visits northern Nigeria, the area of Boko Haram's primary threat, on Friday.