The country's president is promising to find the girls.
Several more U.S. experts are expected to arrive in Nigeria Friday to join colleagues already on the ground assisting the Nigerian government in its rescue operation. The missing schoolgirls have been gone for 25 days.
The British experts are expected to work closely with U.S. officials and agents in the search for the missing girls, the British government said as Boko Haram militants continued to stage attacks in northeastern Nigeria. China and France have also promised help, and the deputy prime minister of Spain, Soraya Saenz de Santamaria, told reporters in Madrid on Friday that her government had decided to make available a specialist police team to assist, if Nigeria approves.
Britain said its aim was not only to help with the current crisis but to defeat Boko Haram.
"The team will be considering not just the recent incidents but also longer-term counter-terrorism solutions to prevent such attacks in the future and defeat Boko Haram," the Foreign & Commonwealth Office said in a statement Friday.
U.S. intelligence believes that many of the girls are being held in small groups and may have been taken into neighboring countries, specifically Chad and Cameroon, CBS News correspondent Margaret Brennan reports. That's why the U.S. embassies in both of those countries are coordinating with the U.S. team in Nigeria.
A Pentagon team of around 50 security experts who were already stationed in Nigeria have been assisting the government, Brennan reports. Soon, the FBI and the Justice Department are expected to join them.
There are restrictions on how much help the Pentagon can give to the Nigerian military because it is considered a human-rights violator.
This is a Nigerian-led operation, which means the U.S. needs the Nigerian government's permission to operate, Brennan reports. Part of these first few days is simply figuring out what the Nigerians need and then what the U.S. will be allowed to do. At this point, U.S. troops will not be used in any rescue operation.
Securing the release of the girls is not going to be easy. Activist Shehu Sani has brokered two previous peace deals with Boko Haram. Both of them failed.
These are not men who can be easily reasoned with, he told CBS News' Debora Patta. But he believes the girls could be used as a prisoner swap for Boko Haram militants being held in Nigerian jails.
"Right now in detention we have some high-ranking members of Boko Haram," said Sani. "They could be used at a very crucial hour like this. The Boko Haram group would not let these girls go until and unless you have a clear deal, whether it's by force to freedom or whether it's through negotiation."
On Thursday the government of Borno state, where the school is located, identified 53 girls who escaped, potentially subjecting the girls to stigma in this conservative society.
The government said in a statement that the 53 girls it identified by name include those who fled the day they were kidnapped and those who escaped from Boko Haram camps days later.
Borno's government did not explain the decision to name the girls.
It is well over three weeks since the abduction. The girls had put in a hard day studying for their exams, but their hopes for the future dissolved when members of the radical Islamic group broke in, set fire to the school and herded the girls into trucks.
A mother told Al-Jazeera English she would do anything to see her daughter again.
"Even (if) they take my life at that moment, I think I'm satisfied, more than when they take my daughter away," she said. "Honestly, I'm not happy at all. I just feel like killing myself."
Meanwhile, a local government official confirmed to The Associated Press that the Islamic extremists bombed a bridge linking the town of Gamboru to the Borno state capital, Maiduguri, the headquarters of the Nigerian military offensive. Gamboru was attacked on Monday by Boko Haram, leaving many dead. Estimates of the death toll from that attack ranged from 100 to as many as 300. Local security officials said on Friday that Boko Haram militants bombed the bridge as they retreated following the attack on Gamboru's main market, where at least 50 bodies have since been discovered from the debris of burned shops.
Communications with the remote town are difficult and it was not immediately possible to reconcile conflicting accounts of when the bridge was bombed. One account said Monday while another said Thursday.
Local traders in Gamboru said Friday that their businesses were suffering, with trailers and heavy trucks now stranded on either side of the damaged bridge.
"We are in trouble," said Gamboru resident Mamman Abu.
The bombing of the bridge also prevents army convoys reaching Gamboru while leaving the way open for the insurgents to escape across a strategic bridge into neighboring Cameroon - a bridge leading into mountains where the militants are known to have hideouts in caves.