The United States says it is sending about 2,000 extra troops to Iraq and asking 3,500 more to stay on longer.
The US begins a massive rotation of personnel in the New Year
Pentagon officials said the moves were designed to maintain capabilities over the next few months.
The US military is to begin a massive rotation of forces at the start of next year that will see all 123,000 troops now in Iraq replaced by fresh units.
The additional troops will comprise an extra brigade of the elite 82nd
The soldiers will be leaving in January and be there for about four months.
More than 3,500 troops also from the 82nd Airborne Division who are in Iraq now and were due to return home in February are being asked to stay on an extra couple of months.
Pentagon officials are portraying this as a minor adjustment to their planning, says the BBC's Nick Childs, at the Pentagon.
But it is a reflection of how seriously they still take the security situation in the country and it clearly won't be welcome news to the troops who are affected or their families, our correspondent adds.
General John Abizaid, commander of the US Central Command, asked for the deployment of extra soldiers after deciding a brigade of the Washington National Guard that was set to deploy in January needed more training, Pentagon officials told AFP news agency.