Michelle Obama has been actively campaigning for the release of the girls. Photo: BBC
US First Lady Michelle Obama is to deliver her husband's weekly presidential address to condemn last month's abduction of Nigerian girls.
First ladies normally refrain from outspoken foreign policy remarks, but Obama has been a vocal campaigner for the release of more than 200 girls.
Fifty-three of the girls escaped soon after being seized in Chibok on 14 April but many more remain captive.
Islamist militant group Boko Haram claimed responsibility for the attack.
Nigeria's military had advance warning of the raid on Nigeria's north-eastern state but failed to act, Amnesty International claimed on Friday.
Nigeria's authorities say they "doubt the veracity" of the human rights group's claim.
'CALL TO ACTION'
Obama is due to make the address ahead of Mother's Day, which the United States marks on Sunday.
"As the mother of two young daughters, Obama is taking up the opportunity to express outrage and heartbreak the president and she share over the kidnapping," White House Deputy spokesman Eric Schultz said.
"The first lady hopes that the courage of these young girls serves as an inspiration... and a call to action for people around the world to fight to ensure that every girl receives the education that is their birthright", he added.
Michelle Obama has often appeared alongside her husband during the weekly addresses, which is broadcast on radio with a video version available online, but it is the first time she will deliver the speech alone.
President Obama has described the kidnapping as "heartbreaking" and "outrageous".
A small number of US and British experts have now started arriving in Nigeria to assist the government's rescue efforts.
Nigerian President Goodluck Jonathan said on Friday he believed the schoolgirls were still in his country and had not been moved to neighbouring Cameroon.
Boko Haram has admitted capturing the girls, saying they should not have been in school and should get married instead.
In a video, group leader Abubakar Shekau threatened to "sell" the students.
Boko Haram, whose name means "Western education is forbidden" in the Hausa language, began its insurgency in Borno state in 2009.
At least 1,200 people are estimated to have died in the violence this year alone.