Mr Brown said that he had tried to alert world leaders to the seriousness of the situation three weeks ago when it first took place, but his pleas fell on deaf ears.
World leaders only began to get involved this week following a global outcry over the fate of the 276 girls, who were taken by militants from the Boko Haram faction deep into the jungles of northen Nigeria on April 16.
Nigerian President Goodluck Jonathan has pledged to find the girls, saying their rescue would mark "the beginning of the end of terrorism" in the country, and the US announced it was sending personnel and equipment to help Nigerian security forces in their search.
David Cameron has agreed to send a small team of British experts to Nigeria to offer assistance with planning, co-ordination and advice while China has offered help and France said it was sending in a "specialised team" to aid search and rescue operations.
But Mr Brown said that the effort might be too late and that it may be impossible to find the girls.
He said: "I'm really sorry because three weeks ago I tried to alert the world, I said 'look, we need international support to help locate these girls' and I fear they have been dispersed across Africa.
"We may never hear of them again. My message to the families is, we will now do everything in our power, even though it's late, we will do everything in our power to try to locate these girls.
"We will try to get these girls home. The international surveillance and satellite coverage and perhaps air support is designed to make it possible that we do everything in our power, to make sure no girl has to go to school again in fear that she will be trafficked, simply for being in a classroom."
Militants claiming to be from Boko Haram, who have been responsible for at least 5000 deaths, released a video this week where they claimed the girls would be sold into slavery.
The insurgents, whose name means 'Western Education is Forbidden', have been fighting to establish a state run on strict Islamic lines similar to the former Taliban of Afghanistan for years.
As many as 300 people were killed in an assault late on Monday on the town of Gamboru Ngala on Nigeria's border with Cameroon. The extremists opened fire on a marketplace bustling with shoppers taking advantage of the cooler night-time temperatures in the semi-desert region, then rampaged through the town for 12 hours, setting houses ablaze and shooting those who tried to escape.
Mr Brown said that the time had now come for the international community to deploy its resources and take action against them to prevent further acts of kidnap or atrocities.
He said: "It's now time to go beyond the words of comfort. We have to extend the search for these girls now beyond Nigeria into Cameroon, into Chad, into Niger.
"The air support and surveillance and the satellite coverage has now got to go into a number of other states to find the girls.
"In the last few weeks seven teachers were murdered and 27 of their families were kidnapped as well, so we've got to make the schools safe for pupils and their teachers."
He added: "I believe we're in the middle of a civil rights struggle. Girls across the world themselves are leading the way in saying 'we want our education, we want to get qualifications' and it's the duty of all of us to support what is really one of the great civil rights struggles of our generation."