Three journalists jailed for over a decade have died in prison in Eritrea, the world's worst country for press freedom, rights group Reporters Without Borders said Thursday.

Dawit Habtemichael, Mattewos Habteab and Wedi Itay were reported to have died by former prison guards who have since fled the secretive Red Sea state, RSF said.

"In Eritrea, journalists have been thrown in prison just for daring to express their opinions," RSF said in a statement. "Some have been held for more than 10 years and are disappearing one by one."

All three had been held since late 2001, arrested in a draconian purge by Eritrea's authoritarian leader Issaias Afeworki, when he shut down independent media.

A fourth journalist, whose identify could not confirmed, is also feared to have died in prison, the group added.

The journalists were held in the notorious desert prison of Eiraeiro, in the north-east of Eritrea, which RSF calls a "death camp" and an "African gulag."

Eritrean officials could not be reached for comment.