"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."
The outlawed militant group Boko Haram claimed responsibility earlier for the April 14 abduction of some 276 teenage girls from their boarding school in northeastern Borno state.
And the group's shadowy leader, Abubakar Shekau, threatened to sell them into slavery in a 57-minute video obtained by AFP that the State Department said appears "to be legitimate."
There has been mounting anger and frustration in Nigeria at the failure of President Goodluck Jonathan's administration to find the girls aged 16 to 18, leading to protests on the streets.
"Some of the family members armed only with bows and arrows to fight terrorists armed with assault rifles rode into the forests on motorcycles to try to find their girls," said Klobuchar.
Six US senators have introduced a resolution supporting the Nigerian people and calling for the immediate release and return of the girls.
"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.
He called the Boko Haram kidnapping "an affront to the civilized world."
State Department deputy spokeswoman Marie Harf said Washington worries many of the girls have been moved out of the country, after local officials in northeastern Nigeria told AFP the girls had likely been taken to nearby Chad or Cameroon.
Fifty-three of the girls managed to escape from the militants but 223 were still being held, Nigerian state police said Friday.
"We have many indications many of them have likely been moved out of the country to neighboring countries," Harf told reporters.
"We will continue working with" the Nigerian authorities, she added, refusing to outline specific US help.
- 'Endemic problem of corruption'-
In general, Washington has been providing Nigeria with "counterterrorism assistance" in the form of intelligence sharing as well as developing their forensics services, Harf said. But she dismissed suggestions that Washington would deploy military assets on the ground.
Undersecretary of State Sarah Sewall is on her way to Nigeria and will meet with senior officials in the coming days to discuss "this despicable incident," Harf said.
"We're standing by to help in ways that we think are appropriate and that we can."
President Barack Obama has been briefed by his national security team on the kidnapping, which White House spokesman Jay Carney called "an outrage and a terrible tragedy."
Republican Senator John McCain said Washington should "use every asset we have to find these young women, young girls. It's horrifying."
But he argued the abduction was an "endemic" problem of "dysfunctional and corrupt governments... particularly in Nigeria, where you've got a great deal of oil wealth in the country, and none of it has reached the average citizen."
Other senators also focused attention on the horrors of human trafficking, which they said needed to be combated both in the US and abroad.