Two months after he went missing from the Dadaab refugee complex, Abdullahi Mohamed called his mother, Ubah, from a detention centre in Libya where he was being been held by armed gangs. The men asked his mother to pay a ransom of up to $10,000 (£7,850) for the 19-year-old.

Relieved but distraught, Ubah started fundraising for his release, talking to family members in the diaspora and in Somalia.

“I managed to get some money but it is far less than they demand,” she says. “I will continue to contact relatives so that we can secure his release as soon as possible.”

Abdullahi was in his second year of secondary school when he went missing. He spent most of his time in the camp playing football.

“He loves football and dreams of playing in the English Premier League,” Ubah says. “He used to tell me that he would become a celebrity one day and that I would be overwhelmed by journalists looking for his whereabouts.”

Abdullahi is among a handful of young men who left Dadaab last year and were smuggled to Libya through Sudan, according to camp leaders.

Abdullahi told his mother he had left with a group of young Somalis from the camps, some of whom are yet to be connected with their families.

Refugees in Dadaab say the travel ban imposed by Donald Trump, which affected refugee resettlement programmes, has had a devastating impact on the hopes of young people in the camps. Many had been waiting for years only to be told they cannot travel.

Ahmed Dhake is among several hundred refugees in Kenya who were stopped from travelling to the US in 2017. The thought of illegal migration occurred to him, but family ties held him back. “I could not leave my elderly mother behind,” he says. “I am not surprised to see people using the illegal route to escape Dadaab, because the legal pathways are not working.”

The uncertainty surrounding the future of the camp, the closure of which was announced by the Kenyan authorities in May 2016, and the lack of prospects in Somalia make it easier for smugglers and traffickers to exploit them.

“It is a worrying trend that young people from Dadaab are giving themselves up to smugglers,”, says Abdullahi Osman, one of the camp leaders. “They have no other way out; Somalia is not yet safe and Kenya does not want us, so it is not easy to convince them to stay.”

It is a desperate, dangerous journey for refugees, but a lucrative business for people smugglers. The journey, popularly known as tahriib in Somali, is common in the Horn of Africa.

Smugglers recently started operating a new form of tahriib, a “leave-now-pay-later” scheme that enables young refugees to travel without their parents’ knowledge or consent.

A network of people finance the journey, with different groups assuming responsibility for the refugees along the route to Libya. What starts as a mutual agreement often ends in captivity.

Illegal migrants at a temporary detention centre after being detained by Libyan authorities in Tripoli. Photograph: Ismail Zetouni/Reuters

Ubah says she was asked to give varying amounts of money to different people in Nairobi and Mogadishu. She believes these people covered her son’s travel costs.

“My son was a student, he had no means of income, they paid for his expenses and sold him out to thugs in Libya,” she says.

Last August, another mother, Nadifo Abdi, from Ifo camp, whose son is being held with Abdullahi, used social media to plead for help to raise $9,000 for his release. Crying in a Facebook video, posted by a local journalist, she described how she hoped her 20-year-old son would finish school and give back to the family.

“He told me he is being tortured and starved. They punish him because we delayed paying the money they asked for,” she says.

Nadifo’s son was also a student, among the 10,000 children currently enrolled in the camps’ seven secondary schools. But the Kenyan government does not allow refugees to work and so there is little hope of getting employment or going to university. Globally, only 1% of young refugees attend university.

More than 250,000 people now regard Dadaab, in north-east Kenya, as their home. The vast majority are from Somalia.

Since 2014, about 80,000 have returned to Somalia through a voluntary repatriation programme, sponsored by the UN refugee agency. The security situation in Somalia remains dire, with deadly attacks taking place in the capital almost every week. In early November, multiple bomb blasts killed 52 people and injured more than 100 others.

Shrinking humanitarian funding means many aid agencies have either left Dadaab or scaled down their services. The need for food remains pressing, yet even the basic rations upon which refugees depend have been reduced by up to 30%.

The few who manage to escape the camps and reach Europe are stuck in limbo, waiting for a chance to cross to travel on elsewhere.

Abdi Mohamed, 30, left the camps about three years ago. “I was detained in Libya for almost 18 months but finally I made it to Rome, where I am currently living in limbo,” he says. “I sleep rough and depend on little meals from charities. I never thought life would be so difficult in Europe, but I will not give up until I reach the UK or Norway, or anywhere better than Italy. There is no going back.”

The EU is supporting Libyan coastguards to stop migrants from crossing the sea. Those intercepted are returned to overcrowded detention centres in Libya, where they face torture, extortion and abuse.

No one knows exactly how many refugees from Dadaab are currently being held in Libya or have died trying to reach Europe.