BAGHDAD — Three Iraqi men herding cattle were killed Saturday after wandering into the middle of a U.S.-Iraqi mortar training exercise north of the Iraqi capital, the U.S. military said Saturday.

American forces were conducting a live-fire training exercise with Iraqis near Taji, about 12 miles (20 kilometers) north of Baghdad, when the men walked onto the artillery range, the military said. An 11-year-old boy was also injured and evacuated to a U.S. military hospital where he was in stable condition Saturday night, the military said.

The accident comes at a time when the U.S. military is shifting its primary role in Iraq from combat to training Iraqi security forces.

As part of the transition, the U.S. military said Saturday it will have reduced combat power in Iraq at the end of the month by the equivalent of two brigades, or roughly 8,000 troops, meeting the first target in President Barack Obama's withdrawal plan.

Obama has ordered the removal of all combat troops by the end of August 2010. Under an Iraqi-U.S. security pact, the remaining 35,000 to 50,000 troops in training and advising roles in Iraq after that would leave by the end of 2011.

The limited withdrawal of two combat brigades is taking place as a spate of bombings generates fear that insurgents could re-ignite sectarian fighting that nearly tore the country apart. The violence has also raised more questions about the ability of Iraqi forces to maintain stability.

The troop withdrawals will reduce combat strength in Iraq from 14 brigades to 12, meaning about 127,000 troops from a high of 135,000 when the pullout plan was announced earlier this year.

The 2nd Brigade Combat Team, 4th Infantry Division, will not be replaced when it returns to Fort Carson, Colorado, at the end of the month, the military said in statement. A handful of Marine regimental combat teams — the equivalent of a brigade — were the first to leave earlier this year and also will not be replaced.

Together those troops number about 8,000 personnel, a military official said.

The official, with knowledge of U.S. troop strength, spoke on condition of anonymity because he was not authorized to release the numbers.

It is part of the president's plan to gradually withdraw U.S. troops at first, leaving most in place for national elections in January.

Iraq's prime minister warned Iraqis on Saturday that insurgents will likely intensify attacks in the run-up to the elections in an attempt to destroy national unity and political stability.

"Terrorists are increasing their attacks here and there because they recognize that we are about to have a political breakthrough," Prime Minister Nouri al-Maliki told Shiite tribal members during a meeting in Baghdad. "We have taken big steps during a difficult period, and there are still more steps to take to overcome the remaining obstacles."

Al-Maliki warned Iraqis there would be "decisive battles" with insurgents between now and the elections when Iraqis will cast ballots for the 275-member parliament and prime minister.

Al-Maliki, a Shiite, has campaigned for re-election largely on the dramatic decline in violence in the last two years.

But bombings have killed hundreds in Baghdad and the northern city of Mosul in recent weeks, marking the worst violence since the U.S. military turned security of cities over to Iraqi troops on June 30.

Despite the recent attacks, security forces Saturday removed concrete blast walls from a major road in the former Sunni insurgent stronghold of Azamiyah as part of a plan to remove most of the barriers from the capital by mid-September to ease congestion and improve the appearance of streets.

The neighborhood was one of the first in Baghdad to have a concrete wall built around it to protect it from attacks but there was no indication the perimeter barrier would be removed.

In northern Iraq, U.S. Senator John McCain met with Kurdish regional President Massoud Barzani to discuss the upcoming elections, Kurdish officials said. McCain, a Republican from Arizona, was in Iraq leading a congressional delegation.

U.S. military commanders have said tensions between Iraq's Arabs and Kurdish minority pose a greater long-term threat to the country's stability than the current insurgency. The two groups have long been at odds over claims on oil and land within Iraq.

Associated Press writers Hamid Ahmed in Baghdad and Yahya Barzanji in Sulaimaniyah contributed to this report.