Donald Rumsfeld said President George Bush had authorised the cuts in US combat troops in Iraq, below the 138,000 level that prevailed for most of 2005.
Although he did not give specific numbers, Pentagon officials said the troop cut would be in the region of 7000 combat troops. A brigade normally consists of 4000 to 5000 soldiers.
Visiting Iraq in the wake of last week's landmark election, Rumsfeld said the decision was mainly due to Iraqi progress on the political, economic and security fronts.
"President Bush has authorised an adjustment in US combat brigades in Iraq from 17 to 15," Rumsfeld said, addressing several hundred soldiers in a military camp east of Falluja.
"The adjustment being announced today is a recognition of the Iraqi people's progress in assuming added responsibility for their country," he said, adding that the US and Iraqi governments would continue to evaluate the troop situation in the coming months.
Further cuts foreseen
Further reductions will be considered "at some point in 2006," after the new Iraqi government is in place and is prepared to discuss the future US military presence, said Rumsfeld.
But he warned that Iraq faced more challenges, including the battle against armed groups.
Rumsfeld warned that Iraq
would face more challenges
Rumsfeld said the reduction would take place by spring next year, taking the number of soldiers to below the 138,000 that were in Iraq before the country's elections last week.
Two army brigades that were scheduled for combat tours, one from Fort Riley, Kansas, and the other now in Kuwait, will no longer be deployed to Iraq.
That will reduce the number of combat brigades in Iraq from 17 to 15.
"The effect of these adjustments will reduce forces in Iraq by the spring of 2006 from the current high of 160,000 during the [Iraqi] election period to below the 138,000 baseline that had existed before the most recent elections," Rumsfeld said.
Rumsfeld aides said details were to be provided later at the Pentagon.
Friday's announcement marks the first time Rumsfeld has said troop levels will dip below the 138,000 baseline.
Rumsfeld's trip follows an eight-hour visit to Iraq on Sunday by Dick Cheney, the US vice-president, the chief architect of the 2003 US-led invasion which toppled the former Iraqi president Saddam Hussein.