Story highlights Former negotiator believes Boko Haram targeted girls to force concessions

Nigerian security chiefs call on those with information about the girls to come forward

Search must expand to Niger, Cameroon and Chad, a U.N. official says

France joins United States, Britain, China in search for girls abducted by group

Nigeria's embattled leader vowed Boko Haram's abduction of hundreds of schoolgirls would be the terror group's undoing, even as authorities admitted Thursday the girls likely have been separated and taken out of the country.

President Goodluck Jonathan's statements come amid mounting international outrage over the mass abduction and the government's largely ineffective effort to subdue Boko Haram

"By God's grace, we will conquer the terrorists. I believe the kidnap of these girls will be the beginning of the end for terror in Nigeria," Jonathan said at the opening of the World Economic Forum meeting in Nigeria's capital city of Abuja.

He also acknowledged the offers of help from the United States, Britain, China and France, all of which have offered help in the weeks-old search for the girls who were snatched in mid-April from their beds at an all-girls school in rural northeastern Nigeria.

But the task of recovering the girls appeared to grow more complicated with news that U.S. intelligence believe the 276 girls have been split up.

JUST WATCHED Village slaughter blamed on Boko Haram Replay More Videos ... MUST WATCH Village slaughter blamed on Boko Haram 02:27

JUST WATCHED Anger grows over 200 missing girls Replay More Videos ... MUST WATCH Anger grows over 200 missing girls 02:01

JUST WATCHED Malala: 'Girls in Nigeria are my sisters' Replay More Videos ... MUST WATCH Malala: 'Girls in Nigeria are my sisters' 01:37

JUST WATCHED Reporter: Uncle was kidnapped in Nigeria Replay More Videos ... MUST WATCH Reporter: Uncle was kidnapped in Nigeria 01:58

JUST WATCHED Obama: 'This is a terrible situation' Replay More Videos ... MUST WATCH Obama: 'This is a terrible situation' 01:00

"We do think they have been broken up into smaller groups," U.S. Navy Rear Adm. John Kirby, the Pentagon press secretary, said.

He declined to detail how U.S. officials came to the conclusion. It is a sentiment that has been echoed by a number of others, who believe the girls already have been moved out of Nigeria and into neighboring countries.

"The search must be in Niger, Cameroon and Chad, to see if we can find information," former UK Prime Minister Gordon Brown, the U.N.'s special envoy for global education, told CNN.

"It's vital to use the information to find the girls before they are dispersed across Africa, which is a very real possibility."

The girls have not been seen since Boko Haram militants abducted them on April 14 from the Government Girls Secondary School in rural Chibok, about 80 miles (130 kilometers) west of Maiduguri and some 600 miles from the capital of Abuja.

That was followed on Sunday night by another kidnapping, with villagers in Warabe accusing Boko Haram militants of taking at least eight girls between the ages of 12 and 15.

Boko Haram leader takes a new tact

"I abducted your girls," he taunted in the video, first obtained by Agence France Presse. "There is a market for selling humans. Allah says I should sell. He commands me to sell."

Shehu Sani, a former negotiator between Boko Haram and the government, believes the group targeted the girls to force concessions from the Nigerian government -- beginning perhaps with the release of its followers from prisons.

"The fact Shekau said he would sell the girls and did not say he would kill them is a clear indication that negotiation is possible. Shekau's video is not going to be the last word from the group on the girls," he said.

Now, Boko Haram may be going after those trying to find the girls. On Thursday, Nigerian police said one officer was shot in the neck during a gunfight with suspected members of the group on the road between Maiduguri and Chibok.

And on Monday, Boko Haram attacked Gamboru Ngala, a remote state capital near Nigeria's border with Cameroon that has been used as a staging ground for troops in the search for the girls. Some of the at least 310 victims were burned alive.

The assault fits a pattern of revenge-seeking by Boko Haram against those perceived to disagree with the group or those who have provided aid to the Nigerian government.

'Time is of the essence'

The Nigerian government has been under fire by those who say government officials failed to take action in the hours and then days after the girls were abducted. Jonathan, who waited three weeks before speaking to the nation on the matter, and his security ministers have defended the response, saying efforts were under way but could not be disclosed publicly.

Nigeria appeared this week to admit it needed help, accepting offers of assistance from world leaders.

The United States was among a number of nations who repeatedly offered assistance to Nigeria in recent weeks, Kirby said.

"In a hostage situation, time is of the essence," Kirby said. "...We lost some time."

The United States is sending a team of law enforcement experts and military advisers. France said Thursday that it would send a "specialized team" to help. The British government is also sending a small team, Prime Minister David Cameron's office said. Neither country said exactly what expertise their teams would bring.

British satellites and advanced tracking capabilities also will be used, and China has promised to provide any intelligence gathered by its satellite network, the Nigerian government said.

Seven members of the U.S. military are scheduled to arrive Friday in Nigeria to join a team of advisers supporting the Nigerian efforts to rescue the girls, Kirby said. Right now, there are no plans to send U.S. combat troops, he said.

Nigeria's top security officials appealed to the public for help during a visit to the Chibok school on Thursday, according to a statement released by Nigeria's director of defense information.

The chief of defense, Air Marshal Alex S. Badeh, called on members of the immediate community to provide security agencies with useful information that will lead to the rescue of the girls.

Boko Haram, which translates to "Western education is sin" in the local Hausa language, has said it wants a stricter enforcement of Sharia law across Africa's most populous nation.

Photos: Photos: Nigerians protest over kidnapped girls Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Police in riot gear block a route in Abuja, Nigeria, on Tuesday, October 14, during a demonstration calling on the Nigerian government to rescue schoolgirls kidnapped by the Islamist militant group Boko Haram. In April, more than 200 girls were abducted from their boarding school in northeastern Nigeria, officials and witnesses said. Hide Caption 1 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Women in Abuja hold a candlelight vigil on Wednesday, May 14, one month after the schoolgirls were kidnapped. Hide Caption 2 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – People march in Lagos, Nigeria, on Monday, May 12, to demand the release of the kidnapped schoolgirls. Hide Caption 3 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Catholic faithful in Abuja take Holy Communion and pray for the safety of the kidnapped schoolgirls on Sunday, May 11. Hide Caption 4 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Catholic faithful attend a morning Mass in honor of the kidnapped schoolgirls in Abuja on May 11. Hide Caption 5 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Catholics nuns pray in Abuja on May 11. Hide Caption 6 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – A woman attends a demonstration Tuesday, May 6, that called for the Nigerian government to rescue the girls. Hide Caption 7 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Community leader Hosea Sambido speaks during a May 6 rally in Abuja. Hide Caption 8 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Brig. Gen. Chris Olukolade, Nigeria's top military spokesman, speaks to people at a demonstration May 6 in Abuja. Hide Caption 9 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Women march Monday, May 5, in Chibok, Nigeria. Hide Caption 10 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – People rally in Lagos on Thursday, May 1. Hide Caption 11 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Police stand guard during a demonstration in Lagos on May 1. Hide Caption 12 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Protesters take part in a "million-woman march" Wednesday, April 30, in Abuja. Hide Caption 13 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Obiageli Ezekwesili, former Nigerian education minister and vice president of the World Bank's Africa division, leads a march of women in Abuja on April 30. Hide Caption 14 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – A woman cries out during a demonstration in Abuja on Tuesday, April 29, along with other mothers whose daughters have been kidnapped. Hide Caption 15 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – A man weeps as he joins parents of the kidnapped girls during a meeting with the Borno state governor in Chibok on Tuesday, April 22. Hide Caption 16 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Mothers weep April 22 during a meeting with the Borno state governor in Chibok. Hide Caption 17 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Four female students who were abducted by gunmen and reunited with their families walk in Chibok on Monday, April 21. Hide Caption 18 of 19 Photos: Photos: Nigerians protest over kidnapped girls Nigerians protest over kidnapped schoolgirls – Borno state Gov. Kashim Shettima, center, visits the girls' school in Chibok on April 21. Hide Caption 19 of 19

JUST WATCHED US offering help for kidnapped girls Replay More Videos ... MUST WATCH US offering help for kidnapped girls 01:44

The militants have even been known to kill Muslim clerics who dare criticize them.

The United States has branded Boko Haram a terror organization and has put a $7 million bounty on Shekau.

Nigerian police also announced a reward of about $310,000 for information leading to the girls' rescue.