BAGHDAD — Eleven American service members have been killed since Sunday in a surge of violence after an Iraqi military offensive against Shiite Muslim militias, the U.S. military said Tuesday. Five died in Baghdad alone in clashes with the Mahdi Army militia of radical Shiite cleric Muqtada al Sadr and in rocket attacks, the military said.

The violence occurred on the eve of testimony before Congress on Tuesday by Army Gen. David Petraeus, the commander of U.S. forces in Iraq, and U.S. Ambassador to Iraq Ryan Crocker.

Sadr threatened Tuesday to end a six-month freeze he's called on his militia's military activities. The freeze was credited with a large part of the drop in violence in Iraq, but in recent weeks an offensive against the militia by Iraqi Security Forces and American forces spiked the violence in southern Iraq and in Baghdad.

"If it is required to lift the freeze (cease-fire) in order to carry out our goals, objectives, doctrines and religious principles and patriotism, we will do that later and in a separate statement," Sadr said in a statement read Tuesday by his aide Salah al Obaidi.

In the same statement, the cleric announced a postponement of a march planned for Wednesday in Baghdad to protest the U.S. occupation of Iraq. Wednesday is the fifth anniversary of the city's fall. Hundreds of men were turned away at checkpoints as they tried to enter the city for the protest.

"I call those beloved Iraqi people who wish to demonstrate against the occupation to postpone their march, out of my fear for them and my concern to spare their blood," Sadr said in the statement. "I fear that Iraqi hands will be lifted against you, although I would be honored if the Americans were to lift their hands against you."