The United States has no plans to send troops to Nigeria to help recover hundreds of schoolgirls kidnapped by Boko Haram, US Defense Secretary Chuck Hagel said in an interview broadcast on Sunday.

"There's no intention, at this point, to (put) American boots on the ground," Hagel told ABC television's "This Week" program.

Washington last week sent a team of military advisers as part of an international effort to help Nigeria find the girls, who were abducted nearly a month ago by the extremist Islamic group.

The US defense chief was far from upbeat about chances of finding the girls.

"It will be very difficult. It's a vast country. This is not going to be an easy task," Hagel said, in the interview recorded Saturday.

"We're going to bring to bear every asset we can possibly use to help the Nigerian government," he said.

Boko Haram, whose name is loosely translated as "Western education is forbidden" kidnapped the girls from their dormitory on April 14, and has threatened to sell them.

The incident has drawn worldwide outrage, including from US President Barack Obama.

First Lady Michelle Obama in a recorded message on Saturday condemned the kidnappings as "unconscionable."