Associated Press

ABUJA, Nigeria -- U.S. reconnaissance aircraft were flying Tuesday over Nigeria in the search for nearly 300 kidnapped schoolgirls, a day after the Boko Haram militant group released the first evidence that at least some of them are still alive and demanded that jailed fighters be swapped for their freedom.

A Nigerian government official said "all options" were open in the effort to free the girls, who were shown fearful and huddled together dressed in gray Islamic veils as they sang Quranic verses under the guns of their captors in a video released Monday.

The footage was verified as authentic by Nigerian authorities, who said 54 of the girls had been identified by relatives, teachers and classmates who watched the video late Tuesday.

"Fifty-four of the girls in the video have been identified by their names in an exercise that involved some parents of the girls, fellow students, some teachers, security men and some officials of the Borno state government," said Isa Umar Gusau, a spokesman for the Borno state governor.

The abduction has spurred a global movement to secure the girls' release amid fears they would be sold into slavery, married off to fighters or harmed following a series of threats by Boko Haram leader Abubakar Shekau.

Protesters marched through the streets of the capital, Abuja, Tuesday to demand more government action to find and free the girls, who are believed to be held in the vast Sambisi forest some 20 miles from the eastern town of Chibok, where they were seized from their school on April 15.

A U.S. reconnaissance mission was being carried out by a manned MC-12 surveillance aircraft, which is based in Niger, according to senior U.S. defense officials in Washington. In addition to the turboprop model that has seen heavy use in Afghanistan, U.S. officials were also considering the use of drones.

Gen. David Rodriguez, head of U.S. Africa Command, was in Abuja on Tuesday meeting with officials at the U.S. Embassy, according to the defense officials, who spoke on condition of anonymity because they were not authorized to discuss the matter publicly.

The Nigerian military said in a statement that Rodriguez visited Nigeria's defense headquarters to discuss U.S. support for Nigeria's campaign against the Boko Haram militants, who have killed more than 1,500 people this year in a campaign of bombings, massacres and kidnappings.

Nigeria's government initially said there would be no negotiations with Boko Haram, but that stance appeared to have been relaxed amid growing public outrage at home and abroad over the failure to rescue the girls.

Mike Omeri, the director of the government's information agency, said all options were being considered, including the possibility of a military operation with foreign help.

In the video, a camouflage-clad Shekau appeared separately from the girls, an assault rifle slung over his chest, and warned menacingly: "I swear to almighty Allah, you will not see them again until you release our brothers that you have captured."

He said the girls, most of whom are Christians, had converted to Islam.

Boko Haram has waged a five-year campaign that has killed thousands.