About half of the nearly 30,000 additional US troops being sent to Iraq have now been deployed.
The rest were either in staging areas in neighbouring Kuwait or on their way, Rear Admiral Mark Fox told a news conference in Baghdad.
In total there are about 140,000 US troops now in Iraq.
Most of the troops are being sent to Baghdad for a security crackdown to curb rampant sectarian violence.
Reinforcements are also being sent to western Anbar province, heartland of the Sunni insurgency.
Admiral Fox cautioned that the success of the Baghdad crackdown, codenamed Operation Imposing Law, would not be seen in days or weeks but in months.
He was speaking after the bloodiest week since the operation began in mid-February.
