NAIROBI: Former child soldiers in war-torn Somalia are being held in prison conditions in foreign-funded camps, "punishing" rather than rehabilitating them, the top U.N. children's envoy said Thursday.

Leila Zerrougui, U.N. envoy for children and armed conflict, said former child soldiers - who foreign donors were funding to rehabilitate their return to society - were effectively being locked up without trial and denied visits from their family.

"They are held in detention without due process," she told reporters in Kenya after a four-day visit to Somalia.

While some centers were treating children well, others were little more than prisons, she said. Some children were assessed to pose little threat to wider society.

Centers must follow "international standards dedicated to rehabilitating and reintegrating them into the society, rather than punishing them," she said.

She singled out the Serendi Rehabilitation Center in the capital Mogadishu - part funded by the Norwegian government - where 55 children including those who once fought for the Islamist Al-Shabab were being held.

Children there were trained in new skills but are blocked from leaving, denied family visits, and have no access to any legal process to challenge their detention, she said.

"It is not a rehabilitation center, it's detention centre, there's no doubt about it," she added.

"Children cannot leave it, even adults or children cannot leave it freely."

War-torn Somalia is awash with guns after more than two decades of conflict, with a 22,000-strong African Union force battling alongside government troops against the Al-Qaeda-linked Al-Shabab.