He also said the US is handing over more facilities to the Iraqi military as part of the withdrawal.
President Barack Obama has decided to remove all combat troops from Iraq by the end of August 2010, with all troops gone by the end of 2011.
But up to now, details of withdrawals have remained sketchy.
There are currently about 135,000 US troops in Iraq, down from 156,000 last June.
Meanwhile, the remaining 4,000 UK troops there will withdraw from the country by July this year, Iraqi officials have said.
British Prime Minister Gordon Brown visited Iraq in December and announced the troops would leave by the end of July.
But he added their mission would already be complete "by the end of May, or earlier".
On January 1 this year, British forces handed over control of Basra airport, its main military base in the south, to Iraqi officials.
British troops withdrew from Basra city last September and since then, they have been training the Iraqi army.
After British troops leave, a small contingent of military advisers is likely to stay on in Iraq.