Protesters spoke out at the Nigerian embassy in Washington, D.C. to express their disappointment in the Nigerian government after an extremist group kidnapped nearly 300 girls on April 15th. (Jackie Kucinich/The Washington Post)

An international uproar mounted Tuesday over the fate of hundreds of Ni­ger­ian schoolgirls abducted by Islamist militants in mid-April, with the Obama administration preparing to send a team of specialists to Nigeria to help recover the missing girls and U.N. officials warning that the kidnappers could face arrest, prosecution and prison under international law.

In Nigeria, U.N. officials reported that a new kidnapping had occurred, with between eight and 11 girls abducted Sunday by armed militants in the northern state of Borno to prevent them from attending school. It is unclear whether the same extremist group was involved in both abductions. The state’s police commissioner denied that any abductions had taken place.

The White House announced that Secretary of State John F. Kerry had called Ni­ger­ian President Goodluck Jonathan on Tuesday morning and offered to send a team including law enforcement and military experts to help his government find and free the roughly 300 girls seized from a school in remote northeastern Nigeria on either April 14 or April 15. Some escaped, but 276 are believed still missing.

A State Department spokesman said that Jonathan, who has been reluctant to move against the militants, “welcomed” the offer. Kerry, in a separate statement, said U.S. officials had delayed action because Jonathan’s government “had its own set of strategies,” but new developments had “convinced everybody that there needs to be a greater effort.”

“It will begin immediately,” Kerry said. “You’re going to see a very, very rapid response.”

In a video released Monday in which he claimed responsibility for the abductions for the first time , the leader of the Islamist group Boko Haram, Abubaker Shekau, referred to the girls as “slaves” and threatened to sell them in a marriage market.

That high-profile, almost swaggering threat intensified a growing outcry as international and domestic rights groups warned that the girls could face severe abuse.

In New York, a spokesman for the U.N. high commissioner for human rights said at a news conference Tuesday, “We warn the perpetrators that there is an absolute prohibition against slavery and sexual slavery in international law.” He said that meant that those responsible could be “arrested, charged, prosecuted, and jailed at any time in the future.”

President Obama, speaking Tuesday about climate change on the “Today” show, also spoke briefly about Nigeria. He called the kidnappings a “terrible situation” and described Boko Haram as “one of the worst local or regional terrorist organizations.” He said Nigeria had accepted his offer of “help from our military and our law enforcement officials” and that “we’re going to do everything we can to provide assistance to them.”

On Capitol Hill, all 20 female U.S. senators signed a letter to Obama condemning the abductions and calling on him to press for U.N. sanctions against Boko Haram, which the administration has designated a foreign terrorist group. The move was led by Sens. Barbara A. Mikulski (D-Md.) and Susan Collins (R-Maine.)

Collins said the comments by Shekau, who said all girls should be married by age 12 and not allowed to attend school, “call out for a vigorous response from all around the world — men and women alike. But I think having the 20 women senators lead the way is the beginning of sending a very powerful signal.”

A social-media campaign called Bring Back Our Girls has gained rapid traction on Facebook and other sites over the past several days. In Washington, about 75 protesters rallied outside the shuttered Ni­ger­ian Embassy on Tuesday morning wearing “Bring Back Our Girls” T-shirts and denouncing the Jonathan government for ignoring the girls’ plight.

“We are tired of the government putting its head in the sand. Girls in Nigeria have the right to be educated and the right to be safe,” said Omolola Adele Oso, 35, a Nigerian immigrant from Bowie and a leader of the peaceful protest. “These girls could be beaten and burned into subservience. The government wants this problem to disappear, but it will not disappear.”

Most of the demonstrators were Ni­ger­ian immigrants, but they were joined by local human rights activists and families. Amy Thomson, 43, of Chevy Chase, said she had come to the rally “because I’m a mother and I would feel the same if my daughter were in danger.”

Thomson was accompanied by her daughter Emma, 11, who said she had been inspired by the efforts of Malala Yousafzai, a teenage activist from Pakistan who was shot and nearly killed by Islamist militants for promoting girls’ education.

“Boko Haram said Allah told them to take the girls,” Emma said. “But I read about Malala, and she said that is not her Allah.”

Boko Haram, whose name means “Western education is evil,” rejects Western culture and seeks to create a pure Islamic state based on strict sharia law. The group has terrorized much of the Ni­ger­ian rural north for the past five years, killing at least 1,000 people in both Muslim and Christian areas.

The Islamic Society of North America condemned Boko Haram on Tuesday, calling its actions “disgusting and un-Islamic.” The Plainfield, Ind.-based organization called on Ni­ger­ian authorities to capture the kidnappers and bring them to justice.

Even though many Westerners and Nigerians are outraged by the militants’ latest predations, the political, regional and religious pressures inside Nigeria are more complex. This helps to explain why Jonathan has tried to play down the kidnappings rather than actively pursue the perpetrators, and why other officials have cast doubt on the crime and criticized protest leaders.

Despite his official welcome of the U.S. offer of assistance, it remained unclear whether civilian and military officials on the ground would cooperate with the Obama administration’s plan to send a team of experts and set up a “coordinating cell” at the U.S. Embassy in Abuja.

One of the factors in play, several experts said Tuesday, is the permanent tension between northern and southern Nigeria. Jonathan is a southerner and many northerners are said to oppose his bid for reelection next year, which may have dampened his eagerness to intervene in the kidnappings there.

Experts said Jonathan was embarrassed by the kidnappings and failed to anticipate the domestic and international uproar they would cause. His wife cast doubt on whether the abductions really happened and reportedly ordered one protest leader arrested.

Another issue is the internal contradictions within the Ni­ger­ian army, which has waged a scorched-earth campaign against Boko Haram but also reportedly includes sympathizers of the group. Both soldiers and militants have been accused of human rights abuses, but none has been arrested or prosecuted.

“The Nigerian army is large and strong enough to be effective in going after the kidnappers, but so far they haven’t,” said one expert on Africa at an international agency who spoke on the condition of anonymity because he is not authorized to speak publicly. Jonathan, he said, “was trying to treat this like business as usual, and suddenly it has turned into a major political debacle.”

Ed O’Keefe and Karen DeYoung contributed to this report.