CHIBOK, Nigeria (CNN) -- As gun-wielding men herded girls from a Nigerian school into seven cargo trucks last month, some made a dash for freedom. One of those girls has shared her story firsthand with CNN.
With fear in her eyes and voice, the young woman, who didn't want CNN to use her name, described the experience.
A man confronted her and told her to get in one of the vehicles, she said. Once in, she dropped down and ran.
"We would rather go than die," the girl said. "We ran into the bush. We ran and we ran."
Officials have said that the Islamist militants from Boko Haram abducted 276 girls from a boarding school in the northeastern town of Chibok on April 14 and that some escaped into a forest.
The girls who remain missing probably have been separated and taken out of the country, some officials have said.
"I abducted your girls," Boko Haram leader Abubakar Shekau said in a video released last week. "There is a market for selling humans. Allah says I should sell. He commands me to sell."
The girl who described her escape to CNN was still shaken up by the events. When asked to describe what her kidnappers wore, she responded: "I feel afraid."
Nigerian President Goodluck Jonathan said Saturday that he was worried about the girls and thanked other countries, including the United States, that have pledged support in finding them.
"We promise the world that we must get these girls out," Jonathan said.
CIA Director John Brennan told the TV network Fusion that the United States is doing "everything we can" to determine the girls' location, a mission President Barack Obama has made a priority.
U.S. and British officials are in Abuja to help Nigeria's government look for the girls, plan rescue missions and advise on ways to subdue Boko Haram. China and France are also helping in the search.
Several senior officials at the U.S. State Department told CNN that Washington offered assistance in the immediate aftermath of the mass abduction, echoing comments last week by Secretary of State John Kerry that the U.S. has been engaged since Day 1. Nigeria turned down the offer until it became apparent that the situation needed a greater response.
Were Nigerian officials warned?
Scrutiny of the Nigerian government's response to the kidnappings has escalated. A report Friday from Amnesty International says authorities knew at least four hours before the attack that Boko Haram was on its way to raid the girls' boarding school in Chibok.
The Amnesty International report alleges that after Nigerian commanders were informed of the pending attack, they were unable to raise enough troops to respond.
A military contingent of between 15 and 17 soldiers and a handful of police officers in Chibok were unable to fend off as many as 200 Boko Haram fighters who stole the girls from their beds, the report says.
The Nigerian government claims it responded with troops, helicopters and airplanes in the immediate aftermath of the mass abduction.
"It is a very painful period for all of us," Nigerian Defense Minister Musiliu Olatunde Obanikoro said. "We've had sleepless nights trying to bring this to an end. Right now, our primary concern is how that can be achieved and not disclose the details of where they are and whether they are in units or they are in one central location."
Amnesty Intenational Secretary General Salil Shetty wrote an opinion piece for Sunday's editions of the Independent newspaper of Britain that said the organization stands by its claims.
Many have died in violence
Nigerian officials have frequently been criticized in the past for failing to prevent Boko Haram's deadly attacks, particularly in the terror group's stronghold of northeastern Nigeria.
At least 2,000 people have died in violence in northern Nigeria this year, Amnesty said. The most recent Boko Haram attack killed at least 310 people in a town that had been used as a staging ground for troops searching for the missing girls.
U.S. Navy Rear Adm. John Kirby, who serves as Pentagon press secretary, said officials believe that the girls "have been broken up into smaller groups" but declined to detail how they came to the conclusion. His sentiment has been echoed by others.
"The search must be in Niger, Cameroon and Chad, to see if we can find information," said Gordon Brown, a former UK prime minister and the U.N.'s special envoy for global education.
But Jonathan believes the girls are still in Nigeria, somewhere in the Sambisa forest.
"If they move that number of girls into Cameroon, people will see. So I believe they are still in Nigeria," he said.
Prime Minister David Cameron promised Sunday that Britain "will do what we can" to help find the girls.
He made the comments as he held a sign bearing the "#BringBackOurGirls" hashtag on the BBC's "Andrew Marr Show."
Cameron and Pope Francis are the latest high-profile supporters of the social media campaign. U.S. first lady Michelle Obama tweeted a photo of herself with a similar poster last week.
The Pope tweeted Saturday: "Let us all join in prayer for the immediate release of the schoolgirls kidnapped in Nigeria. #BringBackOurGirls."
Also Saturday, Michelle Obama condemned the "unconscionable" kidnapping of the girls, saying in the weekly White House address that it was the work of "a terrorist group determined to keep these girls from getting an education."