Iraqi Shia cleric Moqtada al-Sadr has suspended all armed actions by his Mehdi Army for six months in order to remove rogue elements from the militia.
The move comes after 52 people were killed in gun battles in the southern Iraqi city of Kerbala.
The unexpected order also includes attacks on US troops.
Analysts said the test of the suspension order would be whether the fighters obeyed because it was no longer clear how much authority al-Sadr exercised over the Mehdi Army.
Iraqi national security advisers welcomed his suspension order but said it would need to be followed through.
Earlier this year, the Pentagon called the Mehdi Army, which has thousands of gunmen, the greatest threat to peace in Iraq.
