Pakistan's government has blocked the social networking website Twitter because of material considered offensive to Islam, according to officials.
Mohammad Yaseen, the chairman of the country's telecommunications authority, said the website was blocked because Twitter refused to remove material related to a competition on Facebook to post images of the Prophet Muhammad.
Many Muslims regard depictions of the prophet, even favourable ones, as blasphemous.
Mr Yaseen said Facebook agreed to address Pakistan's concerns, but officials have not been able to draw the same assurances from Twitter.
Officials from Facebook and Twitter could not immediately be reached for comment.
Pakistan blocked Facebook for around two weeks in 2010 because of a similar competition.