Iran has blocked access to the popular video-sharing Web site YouTube.com, and a media rights group warned Tuesday that Internet censorship in the Islamic state is on the rise.

Internet users who tried to call up the YouTube site Tuesday were met with the message, "On the basis of the Islamic Republic of Iran laws, access to this Web site is not authorized," which appears on numerous opposition and pornographic Web sites the government blocks.It was not known how long the site had been on Iran's Web blacklist.

The Paris-based media rights group Reporters Without Borders said YouTube had been blocked for the past five days.

Iran's Shiite cleric-run government regularly blocks opposition Web sites, including blogs, and the number of sites that bring up the "unauthorized" message has been increasing over the past year. Western news sites, however, are generally available.

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Videos from the Mujahedeen-e-Khalq and other Iranian opposition groups have been posted on YouTube.com, along with videos posted by individual Iranians critical of the regime.

The site also has Iranian pop music videos, which are frowned upon by the religious leadership.

In its statement Tuesday, Reporters Without Borders warned that "censorship is now the rule rather than the exception" in Iran.

"The government is trying to create a digital border to stop culture and news coming from abroad, a vision of the Net which is worrying for the country's future," it said.

"The Iranian government policy is not an isolated case. It is getting closer and closer to that of the authorities in China, with particular stress being laid on censorship of cultural output," it said.

In October, Reporters Without Borders named Iran as one of the 13 worst culprits for systematic online censorship, along with Belarus, China, Cuba, Egypt, Myanmar, North Korea, Saudi Arabia, Syria, Tunisia, Turkmenistan, Uzbekistan and Vietnam.

Hardliners have severely restricted pro-reform newspapers over the past six years after they blossomed following the 1997 election of reformist president Mohammed Khatami.

Conservatives in the courts shut down many even before Khatami was succeeded by hard-line President Mahmoud Ahmadinejad last year. Some independent newspapers remain, but their criticism of the government is muted for fear of being shut down.