US officials have said they are worried that many of the schoolgirls who were abducted in Nigeria last month may have now been smuggled across Nigeria's borders into other countries which could complicate the search to find them.
The government is still searching for the 276 students, who are aged 16 to 18, who were kidnapped in the northeast of the country by Boko Haram, an armed group designated by Washington as a "terrorist" orgnanisation.
"We cannot close our eyes to the clear evidence of barbarity unfolding before us in Nigeria," said Democratic Senator Amy Klobuchar, who was addressing the Senate.
As anger and frustration escalates in Nigeria at the government's failure to find the girls, six US senators have introduced a resolution calling for action.
"We and our African allies should do everything to help the Nigerian government rescue innocent girls and return them to their families," Senator Dick Durbin, one of the resolution's sponsors, said in a tweet, the AFP news agency reported.
The comments came as Boko Haram, whose name translates as "Western education is sinful", claimed responsibility for the abduction on Monday and threatened to sell the girls.
In a 57-minute video obtained by AFP, Abubakar Shekau, the group's leader, said: "By Allah, I will sell them in the marketplace."
State Department spokeswoman Marie Harf condemned the abduction as "despicable" and said the US was standing by to assist "in any way we think that is appropriate".
However, she declined to outline specific US help and dismissed suggestions that Washington would deploy military assets on the ground.
Under Secretary of State Sarah Sewall is on her way to Nigeria and will meet senior officials in the coming days to discuss the crisis, Harf said.
On Sunday, President Goodluck Jonathan sought help from the US and other countries, including Britain, France and China to find the girls.
Boko Haram stormed the all-girl secondary school in Chibok on April 14, placing the teenagers, who had been taking exams, onto trucks and disappearing into a remote area along the border with Cameroon.