US First Lady Michelle Obama says the mass kidnapping of schoolgirls in Nigeria is an “unconscionable act” of terror against the education of girls.

Speaking in her husband’s place during Barack Obama’s weekly Saturday morning address, Michelle Obama said that this violence “was not an isolated incident … it’s a story we see every day as girls around the world risk their lives to pursue their ambitions”.

“Like millions of people across the globe,” Mrs Obama said, “my husband and I are outraged and heartbroken over the kidnapping of more than 200 Nigerian girls from their school dormitory in the middle of the night.

“This unconscionable act was committed by a terrorist group determined to keep these girls from getting an education – grown men attempting to snuff out the aspirations of young girls.”

On April 14, 276 schoolgirls were abducted in the northeastern Nigerian town of Chibook, and eight more girls were kidnapped from Warabe on May 5.

Three weeks later, 223 girls are still missing.

The Islamist terror group Boko Haram claimed responsibility, and threatened to “sell” the girls into slavery.

“In these girls, Barack and I see our own daughters. We see their hopes, their dreams – and we can only imagine the anguish their parents are feeling right now,” Mrs Obama said.

She praised the courage of the parents to send the girls to school despite the threats, and of the girls going to class knowing “full well the dangers they might encounter”.

The girls were “determined to one day build careers of their own and make their families and communities proud”, she said.

Obama evoked the example of Malala Yousafzai, the Pakistani girl who “spoke out for girls’ education in her community … and as a result, she was shot in the head by a Taliban gunman while on a school bus with her classmates”.

The courage of Malala “and girls like her around the world should serve as a call to action” because “more than 65 million girls worldwide are not in school.

“Yet, we know that girls who are educated make higher wages, lead healthier lives, and have healthier families,” thus boosting “their country’s entire economy”.

US presidents issue a weekly radio address on a subject of their choosing. A video version is also posted on the White House website each Saturday.

Michelle Obama has appeared alongside her husband during the addresses, but this is the first time that she spoke alone.

Earlier this week, she expressed solidarity with the kidnap victims, tweeting a photograph of herself holding a placard bearing the slogan #BringBackOurGirls, the hashtag used as the rallying cry for a viral campaign calling for the schoolgirls’ return.

Seven military officials along with a State Department expert arrived in Nigeria on Friday to help search for the girls. Three FBI personnel and four others were due to arrive Saturday.

President Barack Obama on Tuesday described the kidnapping as “heartbreaking” and “outrageous.”