US planes in hunt for Nigeria girls
The search for missing schoolgirls in Nigeria is being supported by US surveillance and reconnaissance aircraft.
Nigeria's National Orientation Agency said the government will "use whatever kind of action" it takes to free the 276 students kidnapped by militant group Boko Haram.
A spokesman said: "At the moment, because all options are open, we are interacting with experts, military and intelligence experts from other parts of the world. So these are part of the options that are available to us and many more."
The White House said the US team is made up of nearly 30 people drawn from the State and Defence departments, as well as the FBI. It includes 10 Defence Department planners who were already in Nigeria and were redirected to assist the government.
Another seven Defence Department personnel were sent to Nigeria from Africom, the US Africa Command based in Germany, said White House spokesman Jay Carney.
The US is also sharing commercial satellite imagery with the Nigerian government, a senior US official said.
Boko Haram has said the girls will only be freed after the government releases jailed militants.
The group, 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 kidnapping of schoolgirls at a boarding school in north-east Nigeria last month has focused international attention on the extremist group amid outrage that most of the girls have not been rescued.
Nigeria's government, which has repeatedly denied allegations that it was slow to respond to the mass abduction, initially suggested there would be no negotiations with Boko Haram, but that stance could be relaxed.
Amnesty International says Nigeria's military had advance warning of a possible Boko Haram attack before the April 15 kidnappings in Chibok, in the north-eastern state of Borno, but did not react because of their fear of engaging the extremists.
More than 300 schoolgirls were initially abducted, according to police, and they are believed to be held in the vast Sambisa Forest, 20 miles from Chibok. Boko Haram's leader has threatened to sell the girls into slavery.