The chairman of Pakistan's telecommunications authority says the government has blocked the social networking website Twitter because of material considered offensive to Islam, AP reported.

Mohammad Yaseen says the website was blocked Sunday, May 20 because Twitter refused to remove material related to a competition on Facebook to post images of Islam's Prophet Muhammad.

Many Muslims regard depictions of the prophet, even favourable ones, as blasphemous.

Yaseen said Facebook agreed to address Pakistan's concerns but officials have not been able to get Twitter to do the same.

Officials from Facebook and Twitter could not immediately be reached for comment.

Pakistan blocked Facebook for about two weeks in 2010 because of a similar competition.