* Afghan action follows unrest across Muslim world
* YouTube has declined to remove offending film clip
* Actions seen insulting to Islam have sparked riots in past (Adds detail, quotes)
KABUL, Sept 13 (Reuters) - Authorities in Afghanistan have ordered that YouTube's website be blocked indefinitely to stop Afghans watching a U.S.-made film that insults the Prophet Mohammad which has sparked unrest elsewhere, government sources told Reuters on Thursday.
Four U.S. officials including the ambassador to Libya were killed in the east Libyan city of Benghazi on Tuesday night after anger over the film boiled over and there have also been protests against its content in Egypt and Yemen.
"I welcome any decision by the government because it badly affects the minds of young Afghans," a senior government official told Reuters on condition of anonymity.
Another government official, who also declined to be identified, confirmed the ministry had ordered all Afghan Internet providers to block YouTube's site.
Fears the video could provoke riots prompted at least one provider to act before the government's orders were received.
"We have not yet been told, but we blocked YouTube ... to avoid the blood of innocents spilling over. All it takes is one Mullah watching that video, and then he'll preach about it later on, causing chaos", said Farhad Fazi, the president of Internet provider AFSAT.
However, YouTube may remain accessible to some Internet users until Saturday as some of Afghanistan's major servers are based in India and the United Arab Emirates.
An official at the Ministry of Communications said on Wednesday that an order had been issued to block the site until the film was taken down.
YouTube was shut for several hours in Afghanistan but was then restored. Afghan Internet users were still able to view it on Thursday.
The film-related violence in the Muslim world prompted Afghan President Hamid Karzai to postpone a Friday trip to Oslo, the Norwegian Foreign Ministry said, adding that he wanted to remain at home at this time.
In the past, material and actions deemed insulting to Islam have sparked deadly riots in Afghanistan.
The accidental burning of Korans at a U.S. base near Kabul in February brought thousands of protesters onto the streets across Afghanistan for weeks. Dozens of people were killed.
YouTube, the video website owned by Google Inc, has declined to take the film down, saying it fell within their guidelines, but blocked access to the film in Egypt and Libya because of "special circumstances" in those countries.
An Afghan government official said a letter of complaint had been sent to YouTube asking the site to respect Muslim values.
Clips of the movie, posted on YouTube under several titles including "Innocence of Muslims", portrayed the Prophet engaged in crude and offensive behaviour.