US Secretary of State John Kerry has vowed that Washington will do "everything possible" to help Nigeria deal with the armed group Boko Haram, following the kidnapping of scores of schoolgirls.

"Let me be clear. The kidnapping of hundreds of children by Boko Haram is an unconscionable crime," Kerry said in a policy speech in the Ethiopian capital Addis Ababa on Saturday.

"We will do everything possible to support the Nigerian government to return these young women to their homes and hold the perpetrators to justice. That is our responsibility and the world's responsibility," Kerry said.

The US, he said, was "working to strengthen Nigeria's institutions and its military to combat Boko Haram's campaign of terror and violence".

The schoolgirls were abducted by gunmen from the Chibok Government Girls' Secondary School school in Nigeria's Borno state on Tuesday last week.

Nigerian police on Friday said Boko Haram was holding 223 girls of the 276 seized from the school, revising upwards the number of youngsters abducted.

The girls' abduction has triggered global outrage and prompted protests in a number of Nigerian cities, as desperate parents call on the government to secure their release.

More than 200 people also held a rally on Saturday in front of Washington's Lincoln Memorial to bring attention to the girls' plight.

'No effort' to rescue girls

Nigerian mothers on Saturday vowed to hold more protests to push for a greater rescue effort from the authorities.

"We need to sustain the message and the pressure on political and military authorities to do everything in their power to ensure these girls are freed," Nigerian protest organiser Hadiza Bala Usman told AFP.

She said that women and mothers would on Tuesday march to the offices of the defence minister and chief of defence staff "to ask them what they are doing to rescue our daughters".

"We believe there is little or no effort for now on the part of the military and government to rescue these abducted girls, who are languishing in some dingy forest," she said.

Nigeria's information minister, Labaran Maku, said on Friday that Goodluck Jonathan, the Nigerian president, had chaired a top-level meeting with military and security chiefs about a possible rescue mission.

The mass kidnapping is one of the most shocking attacks in Boko Haram's five-year offensive, which has killed thousands across the north and centre of the country, including 1,500 people this year alone.

Boko Haram, an armed group whose name means "Western education is sinful", is fighting what it calls Western influence and wants to form an Islamic state in Africa's largest oil producer country.