Eritrea is routinely labelled by watchdogs as one of the world's worst offenders against human rights, but the Horn of Africa country rejects the allegations and often accuses rights groups of working for foreign intelligence services to undermine its government.
"After several weeks of investigating reports from sources ... and from prison guards who fled the country, Reporters Without Borders has been able to confirm that three more journalists - Dawit Habtemichael, Mattewos Habteab and Wedi Itay - have died in the northeastern prison camp of Eiraeiro," the group said in a statement, adding that all had been held since late 2001.
"Another journalist arrested in February 2009, whose identity has not been established with certainty, has also reportedly died in detention - in his case, in Abi Abeito military prison, near the capital, Asmara."
The group's assertions could not be independently verified.
Lacking independent media and often accused of harassing journalists, Eritrea is consistently ranked among the world's top violators of press freedom.
The European Parliament urged the government last year to release another journalist, Swedish-Eritrean Davit Isaak, and all 11 former Eritrean officials who have been held incommunicado since a government crackdown in 2001.
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Those detained, and accused of conspiring with Ethiopia to topple Isaias, were Vice President Mahmoud Sherifo, Foreign Minister Haile Woldetensae, military chief-of-staff Ogbe Abraha and eight central committee members.
Mahmoud, Ogbe and four other former central committee members have died from illness and heat exhaustion, according to a guard who had worked at the Embatkala and Eiraeiro camps where the detainees were being held before fleeing to Ethiopia in 2010. The temperature in the camps can reach 50 Celsius (122 Fahrenheit).
Eritrea has said nothing about the detainees' condition or whereabouts. Last year it accused the rights group Amnesty International of plotting to incite an Arab Spring-style popular uprising, a claim the group immediately dismissed.