The former Prime Minister, who has been visiting Nigeria in his role as UN special envoy for global education, said the kidnap of more than 200 girls from their school in the north east of the country four weeks ago was "every parent's nightmare".
He called on Boko Haram to publish pictures of all the girls after a video was released by militants claiming to show 130 of the victims.
Mr Brown said: "It is in one way good news the girls were photographed yesterday. That shows that at least the majority of them are still alive.
"I would challenge Boko Haram to publish the pictures of all the girls so we can know all of them are alive.
"I would also call on every religious community to condemn Boko Haram and tell them that they cannot use girls in this way, either using them as sex slaves or threatening to forcibly convert them to Islam."
Mr Brown said there was a "massive amount of surveillance" being done behind the scenes in the search for the girls with support from around the world. He said the Nigerian government was doing everything in its power to find the girls.
He said conceding to a demand to exchange prisoners would mean further kidnaps by Boko Haram.
The former prime minister said: "If we conceded to an exchange, that would mean Boko Haram would just do this again and again and again, knowing that they could have immediate results from doing so.
"If we can track down and locate the girls and then release them, that would be a blow to the efforts of Boko Haram, who have killed almost 5000 people in the last few years."
He added: "This is every parent's nightmare. My children are going to school this morning and we assume, all of us, that our children are going to be safe and they are going to return home without any danger and any risk.
"But now in Nigeria, this is an almost every-week occurrence that children are either being bombed or burned or they are being kidnapped or abducted.
"We must help the Nigerians stop this."
A British team that includes counter-terrorism and intelligence experts has arrived in Nigeria and has held talks with Nigerian counterparts and political leaders. Downing Street said David Cameron had watched footage of the girls released by Boko Haram militants and it underlined his view about the "pure evil" of those involved in the abduction.
The search for the missing schoolgirls is being supported by US surveillance and reconnaissance aircraft.
The White House said the US team is made up of nearly 30 people drawn from the State and Defence departments, as well as the FBI.
Meanwhile, Nigeria's president has asked parliament for a six-month extension of a state of emergency in three northeastern states due to persistent attacks by Boko Haram.
President Goodluck Jonathan said in a letter to parliament the security situation in Adamawa, Borno and Yobe states "remains daunting".