The report also raised questions about Iraqi Prime Minister Nouri al-Maliki's ability to fulfill a pledge made in January to prohibit political interference in security operations and to allow no safe havens for sectarian militias.

Ryan Crocker, the U.S. ambassador in Baghdad, on Sunday called the situation in Iraq "a mixed picture, but certainly not a hopeless one." He noted frustrations among signs of progress, and cautioned against withdrawing troops too soon.

Also Sunday, the Senate's top Republican said that the Iraqis need to step up their own efforts as the U.S. scales back troops in the wake of rising violence in and around Baghdad.

Sen. Mitch McConnell of Kentucky expressed disappointment with the Iraqi government's progress in stemming violence and said the U.S. presence there "will be different in the fall."

"I don't think we'll have the same level of troops, in all likelihood, that we have now," McConnell said. "The Iraqis will have to step up, not only on the political side, but on the military side to a greater extent. We're not there forever."