The move is part of President Barack Obama's plan to end combat operations in August 2010.
'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 US military said in a statement.
Reducing the number of US combat brigades in Iraq from 14 to 12 would cut the number of American troops - currently around 140,000 - by 12,000.
A US combat brigade typically has 4,000-5,000 soldiers.
President Obama plans to pull all combat troops out of Iraq by 31 August 2010, leaving 35,000 to 50,000 support and training troops as Washington shifts its military focus to Afghanistan.
Last month, President Obama ordered 17,000 extra troops to Afghanistan to tackle a growing insurgency there.
Under a US-Iraqi security pact that took effect on 1 January, the US must withdraw all its troops by the end of 2011.
Major-General David Perkins, spokesman for US forces in Iraq, told reporters in a news conference that in coming months 4,000 British troops also would depart Iraq.
