Al-Sadr has made such announcements before, but the current declaration comes only two months before national parliamentary elections. Sadrists hold 40 out of 325 seats in the legislature, making them the largest single Shiite bloc, and hold six Cabinet seats.
In the late Saturday statement, al-Sadr said his move was to "preserve the reputation of the al-Sadr (family)... and to put an end to all the wrongdoings that were conducted, or could be conducted, under their title." It did not explain further.
"I announce here that I will not interfere in politics. There is no political entity that represents me anymore or any position in parliament and government," it said. "Whoever acts against this will be subjected to legal and religious action." He also ordered all al-Sadr political offices to be closed down.
The move did not appear to be expected. Al-Sadr officials could not be reached for contact. The movement announced a news conference, only to cancel it.
Al-Sadr came to prominence in the 2003 US-led invasion that toppled Saddam Hussein, whose regime killed his father and grandfather. He established his Mahdi Army militia that fought the Americans and is blamed by many Iraqis for much of the sectarian violence that raged in Iraq in 2006 and 2007.
Along with the statement, posted on a website commonly used by jihadists, the Islamic State of Iraq and the Levant put up an image of the beheading of one of the soldiers, as well as captured arms and vehicles. They statement also listed other attacks against security forces in and around Mosul, about 360 kilometers (225 miles) northwest of Baghdad.
The authenticity of the statement couldn't be independently verified, but its style was consistent with previous statements. AP For more details, watch Maytham Kassir's full report in the video above