BAGHDAD – Rogue Shiite militiamen with Iranian weapons and training launched three-quarters of the a ttacks 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.

Also Sunday, Defense Secretary Robert Gates acknowledged that political stability in Iraq won’t likely happen before the Bush administration makes its critical September assessment on whether its war strategy is working.

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.

Dissident members of the militia have said they went to Iran for training and armaments and returned to Iraq to join the fight against U.S. and Iraqi troops.

Odierno 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 the September report to Congress, leading to the surge in rogue militia action.

Tehran has denied U.S. allegations that it is fueling the violence in Iraq.

In one sign of U.S. progress against al-Qaida-linked fighters, the U.S. military announced Sunday it had killed the al-Qaida in Iraq mastermind of a bombing that destroyed the famed golden dome of a sacred Shiite shrine in Samarra last year, an attack that set in motion vicious sectarian violence.

The U.S. military announced the deaths of four U.S. soldiers: two during fighting Sunday in Baghdad and two others in separate attacks Saturday in western Baghdad and another area near the capital.

In Washington, Gates declined to predict that a drawdown of U.S. military forces would happen by year’s end. He also cited some progress in reducing violence locally in regions such as Anbar province, a former base of al-Qaida’s activities in Iraq.

“There is a possibility,” Gates hedged, when asked in broadcast interviews if he considered a troop drawdown this year a “good possibility” or would bet on it.

But many Democrats and some Republicans in Congress, restive over the increasingly unpopular war, have made clear that they want a fundamental shift in war policy should the September assessments fail to show clear progress.