Boko Haram's leader said in a new video obtained by AFP on Monday that more than 200 abducted Nigerian schoolgirls would only be released if the government freed militant fighters from custody.

Dozens of women's rights activists took to the streets of the Senegalese capital Dakar on Monday to denounce the kidnapping of more than 200 schoolgirls in Nigeria by Islamist militants.

Members of 20 organisations who had come together near the Nigerian embassy were moved on by police after less than an hour as the protest had not been authorized, but two were allowed into the embassy to submit a letter of protest.

International efforts have widened to trace more than 200 schoolgirls abducted by Islamists militants in Nigeria, while France has called for African leaders to hold a summit focused on the issue.

Israel on Sunday joined the bid to find the 223 schoolgirls abducted by Boko Haram fighters in Nigeria's restive northeast four weeks ago, but Washington said U.S. troops would stay out of any rescue mission.

The United States has no plans to send troops to Nigeria to help recover hundreds of schoolgirls kidnapped by Boko Haram, U.S. Defense Secretary Chuck Hagel said in an interview broadcast on Sunday.

"There's no intention, at this point, to (put) American boots on the ground," Hagel told ABC television's "This Week" program.

Israeli Prime Minister Benjamin Netanyahu on Sunday offered to help Nigeria search for hundreds of schoolgirls kidnapped by extremist Islamist group Boko Haram, his office said.

A statement said that Netanyahu, currently on an official visit to Japan, made the proposal in a telephone call to Nigerian President Goodluck Jonathan.

Niger Delta gunmen have released three Dutch nationals who were abducted a week earlier, the Dutch foreign ministry confirmed on Sunday.

"I can confirm they have been freed," said ministry spokesman Johanne Doornewaard, adding that they were in good health.

The U.N. Security Council on Friday strongly condemned the abduction of hundreds of Nigerian schoolgirls, as the world body's secretary general dispatched an envoy to help find them.

The diplomatic moves came after Amnesty International said that Nigeria's military had been warned of an attack on a school in which more than 200 girls were abducted by Boko Haram Islamists three weeks ago, but failed to act for nearly five hours.

Hundreds of people joined a protest in the traditional stronghold of Boko Haram in northern Nigeria on Friday, accusing the government of indifference to the kidnapping of more than 200 schoolgirls by the Islamists.

The demonstration in Maiduguri, the capital of Borno state in northeast Nigeria, included residents from the remote town of Chibok, where heavily armed Boko Haram fighters abducted 276 girls on April 14, participants told Agence France Presse.

Nelson Mandela's widow Graca Machel has broken a customary mourning period to call on the Nigerian government and international community to do more to find the 200-plus schoolgirls kidnapped by Boko Haram.

"The Nigerian government has a responsibility to protect its citizens, and keep them safe," she said in an open letter, amid allegations that the authorities ignored a warning of the April 14 attack by the Islamist group.

Nigeria's military was warned of an attack on a school at which more than 200 girls were abducted by Islamists but failed to take action, Amnesty International claimed on Friday.

The claim, which the military has denied, came as foreign experts arrived on the ground in Nigeria to help trace the schoolgirls abducted by Boko Haram.