Media rights group Reporters Without Borders on Friday slammed Twitter's announcement that it could now block tweets from appearing in a specific country as "collaborating with the censors".
RSF's head of social media, Lucie Morillon, said the organisation was "very concerned about the decision," which is "bad news for online freedom of expression".
Twitter on Thursday announced in a blog post that it now had "the ability to reactively withhold content from users in a specific country -- while keeping it available in the rest of the world."
The San Francisco-based company said it was entering "countries that have different ideas about the contours of freedom of expression".
Morillon said the move "doesn't just mean cultural adaptation but it means that Twitter will collaborate with the censors, and help them prevent the publication of criticism of the government or denunciations of corruption."
She noted that it remained to be seen how Twitter would implement the censorship.
"Will it wait for a court order? Will a mere phone call from the local police chief be enough to delete a message, as has long been the case in China? Will it work with keywords?"
"China has for years made foreign businesses who want to operate on its soil sign self-censorship agreements, so now Twitter is also submitting itself to this pact," Morillon said.
Twitter said it had not yet used its new ability but "if and when we are required to withhold a tweet in a specific country, we will attempt to let the user know, and we will clearly mark when the content has been withheld."
Twitter co-founder Jack Dorsey recently described China's blanket block on the website as "unfortunate and disappointing".
Explore further: China blocking Twitter, websites: RSF