Manned US aircraft were flying over Nigeria on Tuesday, searching for more than 200 schoolgirls abducted by Boko Haram Islamists after Abuja dismissed a prisoner-swap offer from the militants.

"We have shared commercial satellite imagery with the Nigerians and are flying manned ISR (intelligence, surveillance and reconnaissance) assets over Nigeria with the government's permission," a senior US administration official said on Monday.

The official declined to be named, and it was not immediately clear what kind of aircraft were being deployed, nor where they were based.

Abducted schoolgirls are seen at an unknown location in this still image taken from an undated video released by Nigerian Islamist rebel group Boko Haram. BOKO HARAM HANDOUT VIA REUTERS

Boko Haram, which wants to impose Islamic law on Nigeria, has killed more than 1,500 people this year in a campaign of bombings and massacres.

The group's leader said in a new video on Monday that the girls, whose abduction has sparked global outrage, would only be released if the government freed militant fighters from custody.

Abubakar Shekau made the claim in a 27-minute video, which apparently showed about 130 of the teenagers who were snatched from their school in the remote northeastern town of Chibok nearly a month ago.

The militant leader said the girls in the video had converted to Islam and all were shown in Muslim dress, reciting the first chapter of the Quran and praying at an undisclosed location.

Asked if the government would reject Shekau's suggestion, Interior Minister Abba Moro said: "Of course."

The White House said on Monday that the US team is made up of nearly 30 people drawn from the State and Defense departments, as well as the FBI, including 10 Defense Department planners who were already in Nigeria and were redirected to assist the government.

Another seven Defense Department personnel were sent to Nigeria from AFRICOM, the US Africa Command based in Germany, said White House spokesman Jay Carney.

State Department spokeswoman Jen Psaki said US intelligence experts were "combing through every detail of the video for clues that might help ongoing efforts to secure the release of the girls".

AP-AFP