Warning that “time is of the essence,” the United States announced Tuesday that it would assemble a special team in Nigeria to help that country’s government rescue more than 200 kidnapped schoolgirls before they are sold into slavery or killed.

Secretary of State John Kerry announced that he had telephoned Nigerian President Goodluck Jonathan to urge him to accept American assistance nearly one month after the girls were abducted. Boko Haram, a group the United States has branded a terrorist organization, has claimed responsibility.

“Our embassy in Abuja is prepared to form a coordination cell that could provide expertise on intelligence, investigations and hostage negotiations and to help facilitate information-sharing and victim assistance,” Kerry told reporters at a joint press conference with European Union foreign policy chief Catherine Ashton.

“President Goodluck Jonathan was very happy to receive this offer and ready to move on it immediately. And we are immediately engaging in order to implement this,” Kerry said.

Asked about the delay between the April 14 kidnappings and the U.S. aid, Kerry echoed the frustration of other senior officials in Washington, who have charged that Jonathan’s government dragged its feet.

“You can offer and talk, but you can't do if a government has its own sense of how it's proceeding,” Kerry said. “I think now the complications that have arisen have convinced everybody that there needs to be a greater effort. And it will begin immediately; I mean literally immediately.”

At the White House, press secretary Jay Carney said the U.S. embassy in Abuja was prepared to assemble an interagency team “that could provide expertise on intelligence, investigations and hostage negotiations, could help facilitate information-sharing and provide victim assistance.”

The team “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,” the spokesman said.

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“These girls were captured and kidnapped 22 days ago, and time is of the essence. Appropriate action must be taken to locate and to free these young women before they are trafficked or killed,” Carney told reporters at his daily briefing.

At the State Department, spokeswoman Jennifer Psaki said that the embassy team “could be a combination of personnel on the ground, and if others are needed to be sent in, I'm sure we will deliver that.”

The team would not, however, include elite American commandos like those deployed to aid in the hunt for fugitive warlord Joseph Kony, officials said.

On Monday, Boko Haram claimed responsibility for the April 14 abduction of the girls from their boarding school in Chibok, northeastern Nigeria, and threatened to sell them as “slaves.”

"I abducted your girls," the Islamist group's leader, Abubakar Shekau, said in the 57-minute video obtained by AFP.

The State Department has said that some of the girls have likely already been taken to neighboring countries.

President Barack Obama and Kerry were to discuss the situation during an Oval Office meeting today.

Carney’s comments included language implicitly rebuking Nigeria for its handling of the situation.

“We urge the Nigerian government to ensure that it is bringing all appropriate resources to bear in a concerted effort to ensure their safe return. We are absolutely committed to helping Nigeria, but it is the Nigerian government's responsibility, first and foremost, to maintain the safety and security of its citizens. And we urge the Nigerian government to take action to ensure that it is bringing all appropriate resources to bear in the effort to find them and free them,” he said.

Jonathan has come under mounting pressure over his handling of the crisis, notably the fact that he spoke out publicly for the first time on Sunday and has, U.S. official say, turned down past U.S. offers of help.

Outrage has been building in the U.S. Congress as well, with bipartisan calls on Obama to do more to aid in the rescue of the girls.

On Tuesday, all 20 women in the U.S. Senate sent a bipartisan letter to Obama denouncing the kidnappings and urging him to seek tougher sanctions on Boko Haram.

"In the face of the brazen nature of this horrific attack, the international community must impose further sanctions on this terrorist organization,” said the group, which was led by Senators Barbara Mikulski, D-Md., and Susan Collins, R-Maine.