The Horn of Africa nation Eritrea leads the world in imposing censorship on the media, followed closely by North Korea, Syria and Iran, a journalism group said.
The Committee to Protect Journalists (CPJ) said in a report published on Wednesday that ten countries stand out as censors by barring international media, putting "dictatorial controls" on domestic media and imposing other restrictions.
Rounding out the ten worst censors are Equatorial Guinea, Uzbekistan, Myanmar, Saudi Arabia, Cuba and Belarus.
The report by the committee, a non-profit organisation based in New York, was released to mark World Press Freedom Day on Thursday.
Many of the countries on this year's list also were on the committee's last list, published in 2006.
"In the name of stability or development, these regimes suppress independent reporting, amplify propaganda and use technology to control rather than empower their own citizens," Joel Simon, the CPJ executive director, said in a statement accompanying the report.
"Journalists are seen as a threat and often pay a high price for their reporting.
"But because the internet and trade have made information global, domestic censorship affects people everywhere."
In making its list, the CPJ said its staff evaluated the countries on 15 benchmarks.
They included blocking of websites, restrictions on electronic recording, absence of privately owned or independent media, and restrictions on journalists' movements.
The report said of Eritrea, that "no foreign reporters are granted access ... and all domestic media are controlled by the government".
It said that North Korea, Syria and Iran were "three nations where vast restrictions on information have enormous implications for geopolitical and nuclear stability".
North Korea, which topped the 2006 list, "remains an extraordinarily secretive place," the report said.
It noted, though, that there have been "some tiny cracks" in its censorship, including the opening of an AP news bureau in the capital this year.
The CPJ said censorship "has intensified significantly in Syria and Iran in response to political unrest".
Syria has banned foreign reporters from the country and limited local reporters from moving freely as it uses its military and police to put down a civilian uprising.
Iran, meanwhile, has blocked websites and imprisoned journalists to limit publication and broadcast of information, the report said.
Myanmar, which had the second most repressive censorship regime in 2006, has fallen to seventh place in the midst of democratic reforms that have already led to a relative improvement in freedom of expression.
In 2006, the top ten censored countries were North Korea, Myanmar, Turkmenistan, Equatorial Guinea, Libya, Eritrea, Cuba, Uzbekistan, Syria, and Belarus.