Activists pause during a prayer vigil for abducted Nigerian schoolgirls in front of the Consulate General of Nigeria in Manhattan, New York May 10, 2014.
Nigeria's army has posted two divisions to hunt for 200 schoolgirls abducted last month by Islamist rebels in an attack condemned globally, including by U.S. first lady Michelle Obama on Saturday.
The soldiers are stationed in the border region close to Chad, Cameroon and Niger to work with other security agencies, said General Chris Olukolade, spokesman for the Defense Headquarters.
The government of President Goodluck Jonathan has faced criticism for its slow response since Boko Haram militants stormed a secondary school in the village of Chibok, near the Cameroon border, on April 14, and kidnapped the girls, who were taking exams. Fifty have escaped, but more than 200 remain with the insurgents.
Earlier this month, Boko Haram leader Abubakar Shekau threatened to sell the girls "in the market".
"The facilities of the Nigerian Army signals as well as all the communication facilities of the Nigerian Police and all the services have been devoted into coordinating this search," Olukolade said in a statement.
"The major challenge remains the fact that some of the information given here turned out in many occasions to be misleading. .... Nevertheless, this will not discourage the collaborative efforts that are ongoing," he said.
The air force has flown more than 250 sorties, and a multinational task force has also been activated and surveillance equipment is deployed in support of 10 search teams, he said.
British experts arrived in Nigeria on Friday to advise the government on the hunt and the United States was also sending an interdisciplinary team including military personnel.
Olukolade said later that foreign security forces involved in the search had not conducted any military operations or made any arrests.
Jonathan said on Friday he believed the schoolgirls remained in Nigeria and had not been transported into Cameroon. It was the first indication he had given of their whereabouts.
The attackers were based in the Sambisa area of Borno state, a Boko Haram stronghold near the school where the girls were abducted, he said.
Michelle Obama took the rare step of delivering the weekly radio address of her husband, President Barack Obama, on Saturday to condemn the kidnapping.
"Like millions of people across the globe, 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," she said in the address.
"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," she said.
The U.N. Security Council on Friday expressed outrage at the abduction, demanded the immediate release of the girls and threatened to take action.
Boko Haram's fight for an Islamist state has killed thousands since it erupted in mid-2009 and has destabilized swathes of the northeast of Nigeria, Africa's top oil producer, as well as neighbors Cameroon and Niger.
The global outrage over the attack has shone a spotlight on the rebellion and institutional challenges faced by the government and military just as Nigeria's economy has overtaken South Africa's as the biggest on the continent.
The kidnappings and a broader militant threat overshadowed the World Economic Forum held in the Nigerian capital this week that showcased investment and opportunity in the country.
Human rights group Amnesty International said in a statement, citing multiple interviews with sources, that the security forces had been warned more than four hours in advance of the school attack but did not do enough to stop it. Olukolade dismissed the report as baseless.