There are conflicting reports on whether Iraqi police or Shi'ite militiamen are in control of the Imam Ali Shrine in the city of Najaf.
Iraq's interior ministry says police and Shi'ite religious authorities entered the shrine and about 500 lightly armed militiamen surrendered, leaving the authorities in control of the holy site.
But a top aide to radical Shi'ite cleric Moqtada al-Sadr, as well as some journalists on the scene, say the militiamen are still in control of the shrine.
U.S. military officials in Baghdad and Washington said they cannot confirm reports that Iraqi police had entered the shrine.
Moqtada al-Sadr's whereabouts is not known. Earlier, he agreed to cede responsibility for the mosque to local religious authorities connected to Grand Ayatollah Ali al-Sistani.