The United States has revealed it is flying manned surveillance missions over Nigeria to try to find more than 200 schoolgirls kidnapped by the militant Islamist group Boko Haram.

The US is also sharing commercial satellite imagery with the Nigerian government, officials said.  It comes after militants released a video of about 130 girls, saying they could be swapped for jailed fighters.

Boko Haram seized them from a school in the northern Borno state on 14 April.

"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," said a senior administration official, who declined to be named.

A team of about 30 US experts, members of the FBI and defence and state departments is in Nigeria to help with the search.