WASHINGTON - An international uproar mounted Tuesday over the fate of hundreds of Nigerian schoolgirls abducted by Islamist extremists 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 eight to 11 girls abducted Sunday by armed extremists 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 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 extremists, "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 that 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."

In Congress, 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 traction on Facebook and other sites over the last 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, Md., 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, Md., 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 extremists 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 last 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 Indiana-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 latest predations, the political, regional, and religious pressures inside Nigeria are more complex. This helps explain why Jonathan has tried to play down the kidnappings rather than actively pursue the perpetrators.

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 Nigeria's capital, Abuja.