Barack Obama has pledged the US will do "everything we can" to help Nigeria find hundreds of abducted schoolgirls - as another eight were taken from a village by armed gunmen.
Militants from the Islamist group Boko Haram seized at least 276 youngsters from a school in the village of Chibok in Borno state, north Nigeria almost a month ago.
The brazen April 15 abduction has sparked international outrage and mounting demands for Nigeria to spare no effort to find and free the girls before they can be sold into slavery or otherwise harmed.
Britain has also offered "practical help" as the hunt for the missing girls entered its fourth week.
The pledge by Foreign Minister William Hague came Tuesday, after the children's kidnapper reportedly said he would sell them.
The US President, who described the situation as "heartbreaking" and "outrageous", said the kidnapping could be "the event that helps to mobilise the entire international community to finally do something against this horrendous organisation".
"We're going to do everything we can to provide assistance to them," he said.
"In the short term our goal is obviously to help the international community, and the Nigerian government, as a team to do everything we can to recover these young ladies."
The US is sending military personnel and FBI officials with expertise in hostage negotiations.
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Protests for the missing Nigeria schoolgirls
Boko Haram will "sell your girls in the market", the terrorist leader vowed Monday in a chilling video message to parents of the kidnapped schoolgirls.
"I abducted your girls. I will sell them in the market, by Allah," Shekau said, according to AFP. "We are holding people [as] slaves."