WASHINGTON (AFP) - The United States said Monday that it feared many of the 276 Nigerian schoolgirls kidnapped in northern Nigeria three weeks ago have been smuggled abroad.

State Department spokesman Marie Harf shared Washington's assessment 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.

The Islamist militant group Boko Haram has claimed responsibility for taking the girls from school, and its leader Abubakar Shekau declared his plan to sell them as slaves in a video released Monday.

The State Department believes the video is authentic and is looking at the best ways to help in the search.

"We have many indications many of them have likely been moved out of the country to neighboring countries," Harf said, told reporters as news of the abduction began to climb up the world news agenda.

"We will continue working with" the Nigerian authorities, Harf said, refusing to outline specific US help being provided in this case.

In general, Washington has been providing Nigeria with "counter-terrorism assistance" in the form of intelligence sharing as well as developing their forensics services, she said.

A senior State Department official, Sarah Sewell, is on her way to Nigeria and will meet with senior officials in the coming days to discuss "this despicable incident," Harf said.

President Barack Obama has been briefed by his national security team on the kidnapping which was "an outrage and a terrible tragedy," White House spokesman Jay Carney said.

Six US Senators have also 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 Monday in a tweet, calling the Boko Haram kidnapping "an affront to the civilised world."