Campaigners have warned of fresh efforts in Ethiopia to clamp down on certain types of internet use.
Reporters Without Borders says that the authorities have installed a system to block access to the Tor network - a "hidden" layer of the internet, used to allow anonymous online communications.
Users already face up to 15 years in jail if they use Skype or similar internet call services.
Addis Ababa said it was only illegal to use Skype "for fraudulent activities".
"The Ethiopian government is trying to attack every means of information exchange," Ambroise Pierre from the Reporters Without Borders Africa service told BBC News.
"There's already a very strict control over written press, and last year several journalists were arrested, and now the government is tackling communications over the internet.
"More and more people in Ethiopia are turning to new technologies, and some are even able to bypass censorship, which explains why the government is trying to use effective methods to control internet communications."
But a governement spokesman told the BBC's Anne Waithera in Addis Ababa that Skype was "not illegal" in the country.
"Skype is not illegal. What is illegal is using Skype for fraudulent activities such as making unathorised calls," he said.
"Some people even come from other countries with their sim cards and operate here illegally.
"We need to have regulations, that is standard in all countries. In any business you have to be licensed.
"This law is in the public interest and the right of individuals to communicate is protected by our constitution."
Al Jazeera recently reported that Ethiopia passed a law on 24 May criminalising the use of VoIP (voice over internet protocol) calls. It said the maximum sentence was 15 years in jail.
Other local reports have said that individuals providing such services face sentences of up to eight years, and users could also be imprisoned for using banned social media sites.
The BBC could not independently confirm the details.
While criminalising such acts may be new, Ethiopia has long restricted internet use.
"I used a British internet telephony provider, but [the government eventually] blocked the ports," said Elizabeth Blunt, the BBC's former Ethiopia correspondent who worked in Addis Ababa between 2007 and 2009.
She added that there were two reasons for the authorities to prohibit internet communications.
"Internet cafes may be allowing people to make calls for far less than the cost of Ethiopia telecom, the state's telecommunications provider that has the monopoly and charges very high prices - and doesn't want to have its service undermined," says Ms Blunt.
"But there is also the issue that Skype can't be listened to so easily and can't be controlled."
The Internet Society, an international organisation which promotes equal access to the net, also raised concerns.
"Placing restrictions such as banning VoIP goes against our belief in an open and unencumbered internet," said its vice president for public policy Markus Kummer.
Reporters Without Borders said it was worried the latest effort to block access to Tor might be the first step towards creating a system that would allow the authorities to intercept any email, social network post or VoIP call made in the country.
"This is a country where surveillance is very important - due to years of all the conflicts and political rivalry," said Mr Pierre.
"The opposition and the media are being listened to, and people usually take care when they talk on the phone.
"We've had in the past certain cases of blocking websites of independent and opposition parties, so censorship isn't new - but now it's a new stage, and what Reporters Without Borders is worried about, is that [by criminalising] communications by Skype, the government is implementing a system to have a general policy of internet control."