Boko Haram has been driven out of most of its bases in northern Nigeria but in the past year, it has moved across the border to Chad. The BBC's Thomas Fessy met one family who have been devastated by the Islamist militant group.

Djibrilla, 64, sits quietly in the corner of the hut. Gusts of dusty wind blow through the walls of reeds. Months have passed without a sign of life from five of his eight children.

Aged between two and 15, they were abducted by Boko Haram militants last year.

Where are they? How are they doing? Are they being forced to fight? So many questions he cannot answer.

"I can't understand even though I am trying to make sense of it," he says.

"I am so shocked, they are too young to defend themselves. I have to give it up to God, there's nothing I can do."

The five children were seized during an attack on their village. Those who survived the deadly raid, like Djibrilla, are now living in a makeshift camp.

Many of these camps have sprung up between the sandy tracks and dunes that surround Lake Chad, which forms the border between Nigeria and Chad.

People who lived on the lake islands were forced to settle in the desert, away from their fishing grounds and farmlands.

Boko Haram fighters attacked their homes or they were told to move out by Chadian soldiers who fear jihadis may infiltrate them.

Chad, Nigeria, Cameroon and Niger are among the countries that have finally come together to form a regional task force to tackle the militants, who have staged attacks on all four countries.

Djibrilla's community is obviously not receiving much assistance.

In this desolation, "a day of joy", as he describes it, eventually happened last September.

Trucks from the Chadian army stopped by the camp. Soldiers were bringing his eldest son, Youssouf (not his real name), back to him.

'Promised lots of money'

Youssouf, 26, had left home before his brothers and sisters were seized. He had gone to join the militant group of his own free will.

But he was not the only one.

"Men came to our village and told us we should join Boko Haram," says Adam (not his real name).

He is 16 and he spent two months last year with the group.

He is sitting on his left leg, holding his toes while he speaks. As he looks down, his face shows some kind of embarrassment.

Next to him, Youssouf says: "There was nothing to do at home and we were told we would get lots of money.

"We would get everything we wanted."

Most young people in the region have not had any education, except at Koranic school.

On top of the state neglect, a devastating environmental shift has caused Lake Chad to shrink to a small portion of its original size, pushing families who depend on it further into poverty.

The lack of infrastructure, education and prospects has pushed many into the hands of Boko Haram, lured by the promised gold.

"The drying of the lake is creating socio-economic problems among young people," the regional traditional chief, Youssouf Mbodou Mbami, says.

"They have no activity and Boko Haram is telling them that they have money. So they go join their ranks."

"It has had a very serious impact on them."
'Recited the Koran'

Returning from a visit to the region, the UN Regional Humanitarian Co-ordinator for the Sahel, Toby Lanzer, points the finger of blame in the same direction.

"The extreme violence is the most visible driver of instability and need for emergency relief," Mr Lanzer says.

"But the root of the crisis lies in the abject poverty and environmental degradation which has been plaguing the region for decades.

"It is one of the hardest situations I have seen anywhere," he says.

"The scale of the needs is simply immense."

Back at the dusty camp, Adam and Youssouf describe fighters from Chad who recruited them, proof that the Nigerian insurgency has taken on a regional dimension.

"We were split into groups," Youssouf explains.

"Those who had been there longest were trained with weapons and they would gather to recite the Koran; but we were kept apart."

Both young men said they were under the watch of fighters wearing civilian clothes rather than uniforms, and carrying weapons.

They did not know how long they must have stayed before any training would start.

Under pressure from regional troops, the group was constantly on the move, travelling to different islands on Lake Chad.

For Adam and Youssouf, it soon felt more like a trap than a money-making opportunity.

Out on the lake, it becomes clear that water is all that separates Boko Haram's stronghold in Nigeria from Chad.

The jihadis have been using boats to carry out their attacks, navigating between the many lake islands.

Fear is now ever-present, putting towns and villages on alert.

In the town of Baga Sola, local volunteers are manning a makeshift check-point at the entrance of the market. They have stretched a piece of string across the road to stop passers-by.

"We're like security guards," one of them says. "We search everyone who comes into the market, and the cars must stop here so we can check them too."

People seem to naturally comply with the new order as the volunteers pat them down for weapons.

Last October, four suicide bombers made their way through the crowds and caused carnage in the market. Dozens were killed or wounded.

The community suffered another blow when they discovered that those who detonated the bombs were locals.

'Splinter groups'

Major General Lo Adeosun, who commands the Multi-National Joint Task Force, has come to visit the troops on the ground.

Advancing with a folding seat cane in hand, privilege of hierarchy, he says they have forced the Islamist militants into "splinter groups".

"If you look at it on the grounds of insurgency and terrorism, you can see that the insurgency aspect of it is defeated," he says.

"But the terrorism, we are still battling with."

The soldiers have indeed retaken territory from the jihadis but Boko Haram still has the power to strike the nations around Lake Chad. They are vulnerable from within.

"The money never came so we found a way out," Youssouf says.

The Chadian army kept him in custody for a month after he surrendered to their barracks. Only then, soldiers took him back to his father.

"I was so happy when my son came back," Djibrilla says, "but I warned him: you must stay with us even though we live in poverty."

The community that both Youssouf and Adam left behind has offered them a second chance but the terms are clear; they have their place and they must accept it.

But Djibrilla will not know peace until his five other children return home.