BAUCHI, Nigeria - One of the teenagers who escaped from Islamic extremists who abducted more than 300 schoolgirls says the kidnapping was "too terrifying for words," and she is now scared to go back to school.

Sarah Lawan, a 19-year-old science student, spoke Sunday as Nigerians prayed for the safety of the 276 students still held captive.

Their prayers were joined by Pope Francis.

"Let us all join in prayer for the immediate release of the schoolgirls kidnapped in Nigeria," he tweeted.

Lawan told the Associated Press that more of the girls could have escaped but they were frightened by their captors' threats to shoot them. She spoke in the local Hausa language in a phone interview from Chibok, her home and the site of the mass abduction in northeastern Nigeria.

The failure to rescue those who remain captive four weeks later has attracted mounting national and international outrage. Last week, Nigeria was forced to accept international help in the search, after ignoring offers for weeks.

More experts are expected in Nigeria to help rescue the girls, including U.S. hostage negotiators and others from Britain, France, China, and Spain.

"I am pained that my other colleagues could not summon the courage to run away with me," Lawan said. "Now I cry each time I come across their parents and see how they weep when they see me."

Police say 53 students have escaped. Nigeria's homegrown Boko Haram terrorist network is threatening to sell those who remain in captivity into slavery.

In churches across the nation, Nigerians prayed for the girls, whose plight has brought together ordinary people in a year that had seen growing dissension between Muslims and Christians, disagreements exacerbated by the increasingly deadly attacks of the Boko Haram terrorist network. Africa's most populous nation, with 170 million residents, has almost equal numbers of Christians and Muslims.

The Rev. Stephen Omale prayed at a church in Abuja, the Nigerian capital.

"Wherever they are, God will bring them out in his own mercy, he will see that they are brought out safely, without harm, and also that this act will bring an end to all those who are perpetrating these acts," he told congregants.

U.S. Defense Secretary Chuck Hagel cautioned that it is "going to be very difficult" to find the missing girls. In an interview with ABC's This Week that aired Sunday, he said: "It's a vast country. . . . But we're going to bring to bear every asset we can possibly use to help the Nigerian government."

The office of Israeli Prime Minister Benjamin Netanyahu said in a statement that the leader spoke with Nigeria's president and told him: "We are ready to assist in locating the girls and fighting the brutal terrorism inflicted on you."