The Iraqi authorities also threatened to arrest or even shoot journalists if they remained in the city, which is the epicentre of the Iraqi revolt.
The decision to restrict reports coming out of Najaf fuels speculation that plans are afoot to storm the mosque where the militants are hiding.
This would be a risky move politically because attacking the mosque could prompt a massive Shia backlash and increased support for Moqtada al-Sadr, the militant cleric leading the revolt.
Last week the new Iraqi government banned controversial Arab satellite TV network Al-Jazeera from broadcasting in Iraq for a month after accusing its reports of inciting criminals and gangsters.
All this comes at a sensitive time for reporters in Iraq. On Thursday it was revealed Sunday Telegraph journalist James Brandon was taken hostage after militants burst into his hotel and shot at him twice.
The kidnappers then threatened to kill Brandon unless the US pulled its troops out of Najaf. But on Friday a videotape was made available, which showed one of the captors confirming Brandon would be released.
The Najaf uprising has prompted hundreds of journalists to come to the Iraqi city to report the events despite the danger.
This first time they have been officially warned to leave the city.
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