The White House said Tuesday the U.S. is preparing to send a team to Nigeria to help the government search for nearly 300 schoolgirls abducted more than three weeks ago by Islamic militants.

The team would likely include military personnel, law enforcement personnel and others with experience in intelligence, investigations, hostage negotiation and victims' assistance, White House spokesman Jay Carney told reporters.

"Time is of the essence," he said, stressing that the kidnappings happened 22 days ago. "Appropriate action must be taken to locate and to free these young women before they are trafficked or killed."

Carney said that Secretary of State John Kerry spoke earlier Tuesday with Nigerian President Goodluck Jonathan, who "welcomed" the secretary's offer to send a team to discuss assistance.

"The president was very happy to receive this offer and ready to move on it immediately," Kerry told reporters at a State Department news conference with European Union foreign policy chief Catherine Ashton. "We are immediately engaging in order to implement this. We remain deeply concerned about the welfare of these young girls."

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It's unclear how soon a team of military and law enforcement personnel might be deployed.

Carney urged the Nigerian government to ensure that it is using all available resources to ensure the safe return of the girls.

State Department spokeswoman Jen Psaki said Kerry and Jonathan discussed the "interdisciplinary team" on their phone call on Tuesday.

"It would include U.S. military personnel, law enforcement officials with expertise in investigations and hostage negotiations as well as officials with expertise in other areas that may be helpful to the Nigerian government in its response," she said.

The development comes as Boko Haram, the group of militants responsible for last month's mass kidnapping, seized eight more girls on Monday.

In a video message apparently made by the leader of Boko Haram, Abubakar Shekau claimed responsibility for the schoolgirl kidnappings three weeks ago. In the video, he called the girls slaves and threatened to "sell them in the market, by Allah."

In the 57-minute video, Shekau said "western education should end." He also said he threatened to "give their hands in marriage because they are our slaves. We would marry them out at the age of 9. We would marry them out at the age of 12."

The kidnappings have sparked anti-government protests in Nigeria and been a source of international embarrassment for the country's president, who has been unable to rescue any of the taken teens.

Of the nearly 300 girls who were abducted, 276 remain in captivity, while 53 managed to escape on their own.

Earlier this year, Boko Haram kidnapped and slaughtered more than 50 teenage boys at a government school in northern Nigeria. Some of the boys were burned alive.

Shekau said in the video his group also plans to attack more schools and abduct more girls.

The Associated Press contributed to this report.