The third of five US extra brigades sent to Iraq last year to help curb spiralling sectarian violence is returning home, the US military said.
The redeployment of about 3,500 soldiers from the 3rd Brigade, 3rd Infantry Division in the next several weeks, is part of a wider plan to withdraw the five "surge brigades", or 20,000 troops, by the end of July.
Gen David Petraeus told US Congressional hearings that progress was fragile and reversible
US President George W. Bush last month suspended further troop withdrawals from July, imposing a 45-day freeze on the remaining 140,000 troops to assess the security situation before considering more cuts.
The brigades are credited with helping to reduce violence in Iraq by 60 per cent since last June, but the US commander in Iraq, General David Petraeus, told US Congressional hearings in April that progress was "fragile and reversible".
The drawdown comes at a time when US forces are locked in daily street battles in Baghdad with Shia militia fighters loyal to anti-American cleric Moqtada al-Sadr.
The fighting has taken a deadly toll on US soldiers, threatened to unravel many of the security gains and drawn US forces deeper into the conflict between the Shia-led government and Sadr.
The five brigades were sent to Iraq to help give Iraqi political leaders time to reach a political accommodation and defuse sectarian tensions. Critics say the government has largely failed to take advantage of this window of opportunity.
The Iraq war is a major issue in the November US presidential election, with Democratic nominees Barack Obama and Hillary Clinton calling for troops to be brought home as soon as possible and Republican candidate John McCain warning of the dangers of withdrawing the troops too soon.