The US defence department said it would keep one unit in Iraq 46 days longer than scheduled and send another unit 30 days earlier.
The Pentagon said troop rotations could be changed even further "based upon changes in the security situation".
A surge in sectarian violence in Baghdad and continuing violence elsewhere in Iraq have delayed Pentagon plans to begin withdrawing troops during the close of the year.
Loren Thompson, a defence analyst at the Lexington Institute, a private research group, said: "The [US] army is coming to the end of its rope in Iraq.
"It simply does not have enough active-duty military personnel to sustain the current level of effort."
Donald Rumsfeld, the US defence secretary, would not confirm the tour extension, but said that from time to time there might be units required to stay in Iraq longer than scheduled.
Rumsfeld also appeared to hint at other adjustments to the troop rotation plan.
"We're also bringing some other units in earlier, which is another way of dealing with that issue of how to keep a sufficient number of troops in Iraq with a limited number of combat brigades available," he said.
Arlen Specter, a US senator, said on Monday the situation in Iraq was "disintegrating" into a civil war.
"My instinct is once the [November] election is over there will be a lot more hard thinking about what to do about Iraq and a lot more candid observations about it."
In August, General Peter Schoomaker, the army chief of staff, took the unusual step of delaying submission of the army's budget plan, arguing that the service required either a much bigger budget than the administration had proposed or relief from some of its worldwide commitments.
Last week, the top US commander in the region said the US military may increase its force of more than 140,000 troops in Iraq until next spring.