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."
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