Shi'ite cleric Sadr keeps Iraq guessing over truce

"If he does not ... there must be exceptions -- of which the most important one is self-defense," said the commander.

The U.S. military has praised Sadr for the truce but has still pursued what it calls "rogue" Mehdi Army militants, who commanders say get funding and weapons from neighboring Iran.

A U.S. official in Baghdad said Sadr had used the past six months to weed rogue elements out of the militia, which has tens of thousands of members in Baghdad and southern Iraq.

But he said tensions remained high with the government and the Supreme Islamic Iraqi Council, which has strong influence with the security forces and local governments in southern Iraq.

Revoking the ceasefire would be a setback for Iraq, the International Crisis Group think tank said in a recent report.

"This would not only a resumption of sectarian killings, but also an escalation in the fratricidal war between rival Shi'ite militias," it said.

Biddle said he believed Sadr was observing the truce because of increased U.S. troop levels and a move by some Sunni Arab insurgents to stop fighting.

"The combination of Sunni ceasefires, the U.S. surge and the misbehavior of his own militia boxed him into a situation where continued warfare was unattractive," Biddle said.

(Additional reporting by Khaled Farhan in Najaf; Editing by Samia Nakhoul)