The Ministry of Mass Media and Information said many reports posted on dissident websites amounted to character assassination of President Mahinda Rajapakse, his ministers and top officials.
"This ministry believes that those who are operating and maintaining these clandestine websites have been doing so to discredit the government, the head of state," the ministry said in a statement.
"Such slanderous publications should not only be discouraged but also acts of this nature would have to be effectively prevented in the interest of the general public."
It said the websites should register with the ministry at the earliest, but did not say which laws were being used against dissident websites. It did not specify penalties for those who fail to comply.
However, the statement came four days after the US embassy in Colombo said that freedom of expression, including unfettered access to Internet news websites, was a basic right which must be respected by Sri Lankan authorities.
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