"Like millions of people across the globe, my husband and I are outraged and heartbroken" over the mass kidnapping by the terrorist group Boko Haram, the first lady said Saturday in the White House weekly address.
President Barack Obama has directed his administration to do everything possible to help the Nigerian government, she said.
U.S. military advisers arrive in Nigeria Complications in the search for Nigerian girls Should the U.S. search for Nigerian girls?
The first lady has joined the President in the past in delivering the White House weekly address, but this was her first time delivering it solo.
Boko Haram kidnapped hundreds of Nigerian schoolgirls in April, an act that has become the focal point of a worldwide social media campaign demanding their return.
Earlier this week, the first lady tweeted a photo of herself with a sign that said: #BringBackOurGirls.
Tough questions
The Nigerian government found itself scrutinized after a new report charged that military commanders knew the terror group was on its way to raid a boarding school in the northeastern town of Chibok at least four hours before 276 girls were abducted.
The findings by human rights group Amnesty International echo accounts of a number of the girls' parents and villagers, who have described to CNN an ineffective military response in the days and weeks after the girls were taken.
President Goodluck Jonathan's government vowed to investigate the allegations even as it defended its military response and questioned the motive behind the accounts.
The President on Saturday pledged to rescue the girls.
"We have support from other countries and I promise the world that we must get these girls out," he said in a televised speech in southern Nigeria at dedication ceremonies for a university.
Boko Haram's leader, Abubakar Shekau, took credit for the mass kidnappings in a video that surfaced this week. He said he planned to sell the girls into slavery.
In a published interview Friday with Al-Hayat, a semiofficial Saudi newspaper, Saudi Grand Mufti Abdulaziz Bin Abdullah Al-Sheikh, a key religious leader in the Muslim world, condemned Boko Haram as a terrorist organization.
He described it as an organization "set up to smear the image of Islam" and said the group is "not right and misguided, because Islam is against kidnapping, murder and aggression."
Not enough troops to respond?
Scrutiny of the government's response has escalated amid international outrage over the mass abduction, with many asking why Nigeria did not mount a larger response or ask for international help.
The Amnesty International report alleges that after Nigerian commanders were informed of the pending attack, they were unable to raise enough troops to respond.
The commanders left a contingent of between 15 and 17 soldiers and a handful of police officers in Chibok to fend off the militants, the group reported.
As many as 200 Boko Haram fighters carried out the Chibok school raid, Amnesty reported, herding the girls out of bed under the cover of darkness after a firefight with the handful of security forces in the town.
The Nigerian government has claimed it responded, with troops, helicopters and airplanes in the immediate aftermath of the mass abduction.
But the father of two of the girls taken told CNN there had been little sign of military help.
He said first learned of the attack in a telephone call from a friend in Chibok, who told him the town was under attack by Boko Haram.
"Pray for us," the friend told the father, whose identity is being withheld out of a fear of possible reprisal by Boko Haram and the government.
Nigerian officials have frequently been criticized for failing to prevent Boko Haram's deadly attacks, particularly in the terror group's stronghold of northeastern Nigeria.
At least 2,000 people have died in violence in northern Nigeria this year alone, Amnesty said. The most recent Boko Haram attack killed at least 310 people in a town that had been used as a staging ground for troops searching for the missing girls.
U.S. and British officials have arrived in Abuja to supplement a U.S. team already on the ground there, according to officials.
They will help Nigeria's government look for the missing girls, plan rescue missions and advise on ways to subdue Boko Haram.
