Iraqi Prime Minister Nouri al-Maliki says the new Baghdad security plan has officially begun.
Mr. Maliki made the announcement Wednesday, a day after an Iraqi general said the country's borders with Iran and Syria will be sealed for 72 hours as part of the crackdown.
Meanwhile, the U.S. military is standing by its assertion that radical Shi'ite cleric Moqtada al-Sadr has gone to Iran. But Sadr aides insist the Shi'ite leader is in Iraq and has reduced public appearances for security reasons.
In other developments, the U.S. military says six U.S. soldiers have been killed in a series of incidents since Tuesday.
Also, U.S. Major General William Caldwell says "sophisticated weaponry" brought down a U.S. Marine helicopter last week in al-Anbar province. The crash killed all seven U.S. personnel on board.
The military acknowledged Tuesday that the helicopter was brought down by hostile fire. Earlier, the military had suggested mechanical failure might have caused the crash.