US 'outrage' as calls grow to help rescue Nigeria schoolgirls
The US says it considers the abduction of more than 200 Nigerian schoolgirls by Islamist militants "an outrage" and is offering help to try to rescue them.
White House spokesman Jay Carney said President Barack Obama was being briefed as his national security team was monitoring developments.
Earlier, a video emerged of the leader of the Boko Haram group saying the militants intended to sell the girls.
They were taken from a school in the northern state of Borno on 14 April.
Their whereabouts remain unknown and there is mounting anger and frustration in Nigeria at the failure of the government to find them.
"We view what has happened there as an outrage and a terrible tragedy," said Mr Carney in a White House briefing.
"The president has been briefed several times and his national security team continues to monitor the situation there closely. The state department has been in regular touch with the Nigerian government about what we might do to help support its efforts to find and free these young women."
He added that the US was offering counter-terrorism help to Nigerian investigators that involved "information-sharing" and improving Nigeria's "forensics and investigative capacity".
UK Foreign Office minister Mark Simmonds said the UK had offered "planning support" to the Nigerian authorities and said his officials were in Washington at the moment to co-ordinate efforts.
He told the BBC's Today programme that it was difficult for the Nigerian government because of the vast geographical area of the north-east.
"The forest area where the girls are rumoured to be being held is 60,000 sq km (23,166 sq miles). It's an area of hot dry scrub forest 40 times the size of London; it's a wild territory, very difficult for land and air-based surveillance operations to take place... you have extremely porous borders with neighbouring countries - Chad, Cameroon, Niger, so there are very serious challenges," he said.
In the US, six senators have introduced a resolution supporting the Nigerian people and calling for the immediate return of the girls.
Senator Dick Durbin, one of the resolution's sponsors, called the kidnapping "an affront to the civilised world".
"We and our African allies should do everything to help the Nigerian government rescue innocent girls and return them to their families," he said in a tweet.
'Instructions from God'
Boko Haram leader Abubakar Shekau sent a video - obtained by the AFP news agency - in which he said for the first time that his group had taken the girls.
Boko Haram, which means "Western education is forbidden", has attacked numerous educational institutions in northern Nigeria.
In the video, Abubakar Shekau said the girls should not have been in school in the first place, but rather should get married.
"God instructed me to sell them, they are his properties and I will carry out his instructions," he said.
Reports last week said that some of the girls had been forced to marry their abductors, who paid a nominal bride price of $12 (£7).
Others are reported to have been taken across borders into Cameroon and Chad.
President Goodluck Jonathan has said everything was being done to find the girls.
Boko Haram analyst Jacob Zenn says the girls, aged 16 to 18, have probably been split into smaller groups and it will be hard to track them.
"Any effort to rescue them will have to be done in a very piecemeal fashion and might take over a decade," he told the BBC's Newsday programme.