Two aides to Grand Ayatollah Ali al-Sistani were killed in shootings within hours, prompting his Basra followers to boycott Friday sermons in protest amid fears that an internal Shiite power struggle was increasingly targeting Iraq's top Shiite cleric.

The U.S. Embassy resumed limited convoy movements with Blackwater USA protection in Baghdad, four days after all land travel by U.S. diplomats and other civilian officials was suspended in response to Iraqi outrage over the alleged killing of civilians by the American security firm.

U.S. Embassy spokeswoman Mirembe Nantongo said the decision was made after consultations with the Iraqi governments and the convoys will be allowed to leave the heavily fortified Green Zone on a select basis.

"All movements supported by the PSDs have to be mission-essential," she said, referring to an acronym for personal security details run by Blackwater and other security contractors protecting Westerners and other dignitaries in Iraq.

A top aide to Prime Minister Nouri al-Maliki conceded it may prove difficult for the Iraqi government to expel Western security contractors.

The aide, who spoke on condition of anonymity because the investigation into Sunday's incident was ongoing, said a way out of the Blackwater crisis could be the payment of compensation to victims' families and an agreement from all sides on a new set of rules for their operations in Iraq.