"Two brigade combat teams who were scheduled to redeploy in the next six months, along with enabling forces such as logistics, engineers and intelligence, will not be replaced," the U.S. military said in a statement on Sunday.
Lowering the number of combat brigades from 14 to 12 will decrease the number of troops by 12,000 from the current level of 140,000, said Maj. Gen. David Perkins, a spokesman for U.S. forces in Iraq, Reuters reported.
President Barack Obama is planning to remove all U.S. combat troops from Iraq by Aug. 31, 2010 and leave an estimated 35,000 to 50,000 support and training troops in the country.
Under a security agreement negotiated under the Bush administration, the United States must withdraw all its troops from Iraq by the end of 2011.
The drawdown in Iraq is occurring as the number of troops in Afghanistan is increasing.
Last month, Mr. Obama ordered 17,000 more troops to Afghanistan to fight a persistent Taliban insurgency there.
Overall, violence in Iraq has dropped since early 2007, apparently due to several factors, including the involvement of Sunni tribal leaders in a so-called "awakening movement" to counteract al-Qaida attacks, a cease-fire by militias loyal to cleric Muqtada al-Sadr, and a temporary increase in U.S. troops to help enforce security gains.
But hours before Sunday's announcement about the troop reduction, a suicide bomber killed 28 people as recruits waited outside a Baghdad police academy, the first large-scale attack in the capital in almost a month, according to Reuters.
The attack occurred in the morning when a man wearing a belt of explosives rode his motorcycle into a crowd of people waiting at the side entrance of a training center, reported the Agence France-Presse.