General Mark Hertling, a spokesman for the US first armoured division in Iraq, says new US army units have started to arrive to replace his division.
The new units belong to the infantry division. Some of the division's personnel have already arrived and taken up positions around Baghdad airport 35km from the capital city.
"We have around 30,000 people in Baghdad, and there are new units starting to arrive here," said Hertling.
"One brigade has been replaced already. There will be gradual replacement till mid-April, when we celebrate the hand-over of commandment to the infantry division."