The UN's cultural body will on Sunday award its annual press freedom prize to Mazen Darwish, a Syrian journalist and rights activist who has been jailed by the regime for more than three years.

Darwish was arrested on February 16, 2012 along with Hani Zaitani and Hussein Ghreir, his colleagues at the Syrian Centre for Media and Freedom of Expression. They are accused of "promoting terrorist acts."

At a ceremony in Latvia later Sunday to mark World Press Freedom Day, UNESCO will award its prize to Darwish "in recognition of the work that he has carried out in Syria for more than ten years at great personal sacrifice, enduring a travel ban, harassment, as well as repeated detention and torture."

Rights groups, press organisations and the United Nations have repeatedly called for his release, to no avail so far.

Darwish is one of the founders of syriaview.net, an independent news site banned by Syrian authorities in 2006 -- a move he said at the time was part of the state's "repression which targets free expression and democratic activists."

Human rights groups say some 100,000 people have been detained since the uprising against Bashar al-Assad's rule erupted in March 2011, which escalated into an armed rebellion after the regime unleashed a brutal crackdown.

Another 50,000 are believed to be held by the regime's myriad of military intelligence branches.

Created in 1997, the annual UNESCO/Guillermo Cano World Press Freedom Prize honours a person, organisation or institution that has made a contribution to the defence and, or promotion of press freedom anywhere in the world.

It is named in honour of Guillermo Cano Isaza, a Colombian journalist who was assassinated in front of the offices of his newspaper, El Espectador, in Bogota in 1986.