Baghdad ? Rogue Shiite militiamen with Iranian weapons and training launched three-quarters of the attacks that killed or wounded American forces last month in Baghdad, stepping into the void left as Sunni insurgents have been dislodged, a top U.S commander said Sunday.

Attacks against U.S. forces were down sharply last month nationwide, and military officials have expressed cautious optimism that a security crackdown is working. At the same time, the number of attacks launched by breakaway factions of the Shiite Mahdi Army militia has increased, said Lt. Gen. Raymond Odierno, the U.S. second-in-command.

He did not provide a total number of militia attacks. But he said 73 percent of the attacks that wounded or killed U.S. troops last month in Baghdad were launched by Shiite militiamen, nearly double the figure six months earlier.

Odierno said Iran has sharply increased its support for the fighters ahead of a September report to Congress on progress in Iraq, leading to the surge in rogue militia action.

Tehran has denied U.S. allegations that it is fueling the violence in Iraq and the military claims come as the Americans and the Iranians have agreed to set up a committee to deal with Iraqi security issues.

Odierno’s comments hinted at the difficulty Iraqi and U.S. security forces face in keeping the peace once U.S. troops have successfully ousted mostly Sunni al-Qaida-linked fighters from any particular spot.

“We knew this was coming, but there’s been a shift,” Odierno said after touring a mainly Shiite area of southeastern Baghdad. “Because of the effect we’ve had on al-Qaida in Iraq and the success against them and the Sunni insurgency, it’s now shifted.”

Thousands of American and Iraqi troops have flooded the capital as part of a nearly six-month-old security crackdown.