A well-known Eritrean satirical cartoonist has been freed from prison after spending 15 years in detention without charge, his family and friends have told the BBC.

Biniam Solomon, now in his early 60s, was arrested in the capital, Asmara, in 2011 but was never tried. The reasons for his continued imprisonment were not made clear.

Known by his pen name Cobra, Biniam gained fame for his critical and witty cartoons tackling political and social issues.

The authorities have not said why he was set free but it is the latest in a string of similar releases. Nevertheless thousands remain in prison, incommunicado and without trial, in a country long criticised for alleged human rights abuses.

During his 15 years in detention, Biniam had no contact with his family, and received only occasional medical treatment.

According to a relative, he spent the latter part of his detention in Asmara's "crime investigation" prison.

It is widely reported to hold political and conscientious objectors under harsh conditions.

Despite losing an arm in childhood, Biniam pursued a career as an artist and produced a substantial body of work. To supplement his income, he was also employed as a physics teacher at a secondary school in Asmara.

His cartoons were published in several Eritrean newspapers in the four years from 1997.

This was a brief period after independence from Ethiopia when the private media flourished before being being shut down in September 2001. The government closed the private press on the grounds that it was "endangering national security" and a number of journalists were jailed.

Biniam also produced three books that included collections of his work - Subtle is the Ruler, Conversation with Cobra Number One and Conversation with Cobra Number Two.

Examples of his work include the cartoon below where he satirised a period of great uncertainty within the government in 2001 when officials including senior ministers were "frozen out" and recalled without warning:

The minister's wife, on the left, is asking why her husband is not getting up to go to work.

"I might be frozen [suspended]," he responds, adding that he is listening to government radio to find out if he still has a job.

Human rights groups have frequently documented widespread abuses in Eritrean prisons, including the lack of contact with the outside world, inadequate food and medicine, and severe physical and mental suffering. The authorities have rejected the accusations.

Biniam's release comes amid recent reports of the Eritrean government freeing other long-term detainees, though critics note the process remains opaque and outside normal legal procedures.

The UN has previously called for the release of what it said were 10,000 people held without trial in Eritrea.