US forces in Iraq are being bolstered to their highest-ever level in the run-up to next month's election.
Some soldiers have had their tours extended twice
Some units are having their tours extended to cover the expansion, amid fears that violence will increase in the weeks before the vote.
The US is expanding overall troop numbers by 12,000, to reach 150,000 - more than actually invaded the country.
The BBC's Nick Childs at the Pentagon says the move is a sign of continuing concerns about security in Iraq.
The increased total from 138,000 US troops now in Iraq will continue until mid-March, Army Brig Gen David Rodriguez said at the Pentagon.
"The purpose is mainly to provide security for the elections. But it's also to keep up the pressure on the insurgency after the Falluja operation," he added, referring to the recent US-led assault on the Sunni Muslim city.
Some units are having their period of service in Iraq extended from 12 to 14 months, or from seven to nine months.
One unit, the 2nd Brigade of the 1st Cavalry Division, is having its tour of duty extended for the second time. After being slated to fly home in November, then January, they will now remain until March.
But Gen Rodriguez said even if the election was delayed, "they will not be extended
any further than this".
Two battalions of the 82nd Airborne Division are also being sent from the US to boost the numbers.
Our correspondent says the fact that thousands of US soldiers are being asked to stay beyond the Pentagon's self-imposed 12-month limit may re-ignite complaints in the US that the military is being overstretched.
Previously, the largest number of US troops on the ground in Iraq was 148,000 on 1 May 2003, when President George W Bush declared that major combat operations were over.
In other developments, Iraq's president, Ghazi Yawer, on Wednesday insisted next month's election should go ahead as planned.
He is the most prominent Sunni figure to come out in support of the vote date, distancing himself from others who are demanding a delay or a boycott.
Meanwhile, the Islamist group Ansar al-Islam claimed to have abducted and killed
three Iraqis who "collaborated" with the US, in a statement posted on the internet.