U.S. surveillance planes joined an international effort to find more than 200 Nigerian schoolgirls who were abducted by an extremist Islamic sect, U.S. media reports said.
The U.S. provided aircraft over Nigerian territory, and was sharing satellite imagery with the authorities, broadcaster CNN quoted Obama administration officials as saying late Monday.
British and U.S. officials were in the Nigerian capital Abuja to help find the girls, the report said.
An Israeli counterterrorism team was expected to arrive in the country soon, it added.
Members of a Nigerian presidential committee tasked with finding the hostages met Chinese and French representatives on Monday, local newspaper Punch reported on Tuesday.
Islamist group Boko Haram said on Monday it was willing to negotiate with the government over the release of the schoolgirls it abducted four weeks ago.
The group released a video demanding the release of all imprisoned insurgents in Nigeria in exchange for the teenagers. The girls had converted to Islam, the group said.
The government response to the demand was unclear, with one official saying there would be no negotiations, and another from the information ministry saying all options were on the table.