Cardinal John Onaiyekan of Abuja has called for "concrete action" to save the hundreds of girls kidnapped by Boko Haram, saying his country is "ashamed" by the failure to find them.

"Up until now we are hearing practically nothing concrete on the issue," Cardinal Onaiyekan told Vatican Radio May 6. "I think almost every Nigerian is taken aback. We cannot explain what is happening."

"It's still more baffling that our president seems to be impotent. We have to see concrete action."

Nearly 300 girls, most of them aged between 16 and 18, were kidnapped April 14 from their boarding school in Borno, Nigeria's northeastern-most state, by members of the radical Islamist group Boko Haram.

Boko Haram, which means "Western education is sinful," launched an uprising in 2009 and hopes to impose sharia law on Nigeria. It has targeted security forces, politicians, Christian minorities, and moderate Muslims in Nigeria's predominantly Muslim north.

There are 276 girls still in captivity, while 53 escaped, the Associated Press reports.

The schools in the area had been closed due to the threat of Boko Haram; Cardinal Onaiyekan said there was supposed to be security at the school, which had temporarily reopened so that the girls could take final exams.

"The soldiers who were there could not face the group of terrorists that invaded both the town and the school," the cardinal reported.

Boko Haram leader Abubakar Shekau has claimed responsibility for the abductions and has threatened to sell the girls into slavery, threatening more attacks on schools.

A May 5 attack on another town in Borno, in which some 300 persons died, was attributed to Boko Haram. The previous day, another 11 girls were kidnapped in an attack on two villages.

Cardinal Onaiyekan noted the "unprecedented" demonstrations by Nigerians seeking the return of the girls.

The crime has drawn international attention.

U.S. president Barack Obama has said his government will work with the international community and the Nigerian government to do "everything we can to recover these young ladies," he told the NBC television show "Today."

"But we're also going to have to deal with the broader problem of organizations like (Boko Haram) that ... can cause such havoc in people's day-to-day lives."

The president told ABC News that Boko Haram is "one of the worst regional or local terrorist organizations" and has been "killing people ruthlessly for many years now."

The Nigerian government had declined offers of U.S. assistance until May 6, when U.S. Secretary of State John Kerry called Nigerian president Goodluck Jonathan. U.S. military and law enforcement personnel headed to assist the rescue effort include experts in intelligence, investigations, hostage negotiation, and victim assistance, the White House said.

It was announced May 7 that the U.K. will send a small team of experts to assist Nigerian authorities.

Nigerian police have offered a $300,000 reward for information leading to the girls' rescue.

The Council on American-Islamic Relations stated May 5 that "it is almost impossible to express the level of disgust felt by American Muslims at the un-Islamic and obscene actions of the terrorist group Boko Haram for the kidnapping and threat to 'sell' hundreds of Nigerian schoolgirls. We urge authorities in Nigeria and throughout the region to do everything they can to return the girls safely to their families and to eliminate the threat to peace and security posed by Boko Haram."

Boko Haram's attacks have killed thousands since 2009; according to the BBC, they have killed 1,500 in 2014 alone. The U.N. estimates that the attacks have led to more than 470,000 internally displaced persons in Nigeria.

The U.S. recognized Boko Haram as a foreign terrorist organization in November 2013, after a lengthy advocacy effort from human rights and Christian groups.

"We know that Boko Haram have no sense of humanity," Cardinal Onaiyekan said.

"We know that they are killing innocent people. But that they should be able to cart away almost 300 children in the Northeast of Nigeria without any trace of where these children are really baffles us."