An Iranian ayatollah has said that the social networking service Facebook was un-Islamic and being a member of it a sin, the ISNA news agency reported Saturday.

In Iran, it is common for senior clerics to be asked about their stance on certain social issues and whether these issues are compatible with Islamic norms.

Their answers are regarded as a form of decree.

ISNA on Saturday broadcast coverage of the response of Ayatollah Lotfollah Safi-Golpaygani, a senior cleric, to the question about Facebook and Iranian membership in the social networking service.

"Basically, going to any website which propagates immoralities and could weaken the religious belief is un-Islamic and not allowed, and membership in it is therefore haram (a sin)," the ayatollah replied.

"Only the use of websites propagating religious criteria and not leading to any kind of ethical immoralities is of no problem," he added.

According to official figures released last October, 17 million Iranians have a Facebook account, despite heavy restrictions and filtering imposed by the government.

Due to the popularity of Facebook with younger generations, observers believe that the number of real Iranian Facebook users could be much higher than 17 million.

Iran has a population of 70 million, of which than 60 per cent is under the age of 30.

Over 5 million websites are reportedly blocked in Iran, but Iranians use proxy software and virtual private networks (VPN) to access them.

Iranian officials have, for over three decades, been waging what they call a "battle against the invasion of Western culture."

This has led to blockades of "immoral" internet sites and banned Western music and movies. However, pirated versions of those are easily available on the black market.

The country recently established a cyberpolice unit to better police the internet and even plans to introduce its own national internet, though this has been postponed several times.