By Emma Farge

DAKAR (Reuters) - Hundreds of Boko Haram fighters and their families have surrendered in Chad in the past month, security and U.N. sources said, in a sign the military campaign against them is making headway.

Boko Haram, which has killed and kidnapped thousands of people, had seized an area approximately the size of Belgium in northeastern Nigeria by last year but has since lost significant ground amid growing regional military pressure.

Analyst and security sources think the fighters are probably recent recruits that Boko Haram has struggled to retain as it has ceded territory. Defections of Boko Haram fighters have been reported in Nigeria but are not known to have previously occurred on such a large scale.

"They surrendered to our troops on the front line in Lake Chad," said Colonel Mohammad Dole, Chief Military Public Information Officer for the Multinational Joint Task Force (MNJTF) headquartered in Chad's capital N'Djamena.

"The surrenders are taking place because of the firepower of our operations. The groups, many of them armed, have been arriving since September and their number keeps increasing," he said.

Some 240 fighters, most off whom are Chadian, are now being held in detention along with their families, Dole said.

The MNJTF, with troops from Chad, Niger, Nigeria, Cameroon and Benin and intelligence, training and logistical support from the United States, launched a regional operation in July against the group, which has pledged allegiance to Islamic State.

It has since continued patrols around the waterways of Lake Chad - one of the world's poorest regions whose villages were last year regularly struck by fighters, sometimes aboard canoes.

Around 2.6 million people have been displaced in the Lake Chad Basin where Chad, Niger, Nigeria and Cameroon meet.

Signs that regional armies are wresting back control of the Chadian part of the lake is significant since it had been a recruitment hub, even if the group never sought to conquer territory there, said Ryan Cummings, director of consultancy Signal Risk.

"Their presence in Chad was more for recruitment and for resources. Its strikes in the country were punitive," he said, referring to revenge attacks on regional military heavyweight Chad, which has supplied 3,000 troops for the MNJTF.

FORCED RECRUITMENT

Chadian military officials are currently profiling the detainees currently housed at two detention centers in the remote town of Baga Solo, some of whom arrived this week.

Based on previous patterns, it is likely that many were abducted or forcibly recruited by Boko Haram whose name means "Western education is sinful" in the local Hausa language.

Stephen Tull, U.N. Resident Coordinator in Chad, said a total of around 700 people were being held, including men, women and children.

It was unclear how many were fighters, he said. Boko Haram has in the past deployed child soldiers and female suicide bombers.

"They are mostly Chadians and appear to all be more recent recruits," he said citing information from a U.N. visit to the centers earlier this month.

Islamic State named Abu Musab al-Barnawi as Boko Haram's leader in August although another branch loyal to former head Abubakar Shekau is still operational. It was not clear from which branch the fighters surrendered, nor how senior they are.

Philippe Barragne-Bigot, head of the U.N. children's agency in Chad, said that it had set up a center for the children, who he said should be treated as former hostages.

"We want to profile them and make sure they have the right psychological rehabilitation," he told Reuters.

(Reporting by Emma Farge; Editing by Joe Bavier)