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”.
Six US 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.
In an emotional address to the US Senate, Democratic Senator Amy Klobuchar also called for action.
“We cannot close our eyes to the clear evidence of barbarity unfolding before us in Nigeria,” she said.
“This is one of those times when our action or inaction will be felt not just by those schoolgirls being held captive and their families waiting in agony, but by victims and perpetrators of trafficking around the world. Now is the time to act.”
‘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.
The girls were in their final year at the boarding school in Chibok, most of them aged 16 to 18.
In a TV broadcast on Sunday – his first public comment on the abductions – President Goodluck Jonathan said everything was being done to find the girls.