"Part of the problem is that the United States is trying to fight the wrong 'war,'" Anthony H. Cordesman, who holds the Arleigh A. Burke chair in strategy at the Center for Strategic and International Studies, a Washington think tank, said in a statement.

"The United States does need to fight a serious counter-insurgency campaign, but this seems to be focused far too narrowly on both al-Qaida, which is only one Sunni Islamist extremist movement, and on the most radical elements of the Sadr militia," Cordesman said.

"The United States does not have an effective strategy or the operational capability to deal with the broader problem of armed nation-building, or with a widening pattern of civil conflicts," he said.

Sunni insurgent groups in Iraq are believed to also involve many former officers and soldiers of Saddam Hussein's army and other security forces, which were disbanded by the U.S.-led Coalition Provisional Authority. Shiite cleric Moqtada Sadr leads the Iranian-backed Mahdi Army. But it is only one of the network of often competing Shiite militias that have sprung up to control large areas of Iraq in the security vacuum left by the weakness of the central government of Prime Minister Nouri al-Maliki.