Eritrea experiences the worst censorship in the world, while Equatorial Guinea is among the five most heavily-censored countries, says the Committee to Protect Journalists (CPJ).
In a report released ahead of World Press Freedom Day, the committee also notes that Ethiopia and Sudan only just escape being named in the world's "10 most censored countries".
The respected, New York-based lobby names the 10 countries, in order of appearance on the list as: Eritrea, North Korea,  Syria , Iran , Equatorial Guinea, Uzbekistan, Burma, Saudi Arabia, Cuba and Belarus.
Of Eritrea, the CPJ says: "Only state news media are allowed to operate in Eritrea, and they do so under the complete direction of Information Minister Ali Abdu. Journalists are conscripted into their work and enjoy no editorial freedom; they are handed instructions on how to cover events."
It quotes an exiled Eritrean journalist as saying: "Every time [a journalist] had to write a story, they arrange for interview subjects and tell you specific angles you have to write on... We usually wrote lots about the president [Isaias Afewerki] so that he's always in the limelight."
Also noting that Eritrea shuts out the international media, the CPJ adds: "Journalists suspected of sending information outside the country are thrown into prison without charge or trial and held for extended periods of time without access to family or a lawyer. The government expelled the last accredited foreign correspondent in 2007."
Access to the Internet is also restricted in Eritrea: all service providers have to connect to the world through a state-operated company, access is affordable only to a few, and a plan to introduce mobile Internet connectivity was abandoned after the Arab Spring uprisings.
In Equatorial Guinea, the CPJ says the government of Teodoro Obiang Nguema "tightly controls all news and information over national airwaves. Technically, some outlets are privately owned, but none are independent, as Obiang and his associates exert direct or indirect control. State media do not provide international news coverage unless Obiang or another official travels abroad. Censors enforce rigid rules to ensure the regime is portrayed positively; journalists who don't comply risk prison under criminal statutes including defamation."
Here too, the government controls information conveyed abroad: "Security agents closely shadow foreign journalists and restrict photography or filming that documents poverty. The government paid three Washington-based public relations firms a total of US$1.2 million between April and October 2010 to produce positive news about Equatorial Guinea, according to a report by the U.S. Department of Justice."
Although Ethiopia and Sudan escape mention in the top 10 list of offenders, the CPJ notes that in the former, censorship has become tighter in recent years, and that the government appoints media managers and licenses media. Also, "anti-terrorism legislation criminalizes any reporting that the Ethiopian government deems favorable to opposition movements designated as terrorist".
In Sudan, "authorities... frequently confiscate newspapers, which are the widespread form of media. This year, security forces have increasingly adopted the technique of confiscating newspaper editions wholesale to inflict financial losses on publishers".
The CPJ used 15 criteria to make its judgements, including "blocking of websites; restrictions on electronic recording and dissemination; the absence of privately owned or independent media; restrictions on journalist movements; license requirements to conduct journalism; security service monitoring of journalists; jamming of foreign broadcasts; blocking of foreign correspondents."