The Pentagon moved last month to build up the force to deal with uprisings in the south and in Fallujah by extending the tours of 20,000 troops from the 1st Armored Division and the 2nd Armored Cavalry Regiment for at least three months.
Officials, who spoke on condition of anonymity, told AFP those troops will be replaced with fresh units at the end of their extended tours, but the overall force will be maintained at its current strength, which has hovered around 135,000 troops.
General John Abizaid, head of US Central Command, which includes Iraq, "has expressed his desire to keep things at current levels for a while," a senior defense official said.
"I don't think there is going to be a time limit associated with it," the official said.
The plan is expected to be announced on Tuesday, but the brigades chosen for the next rotation in Iraq will probably not be identified all at once, the official said.
"There is a decision in the works. The details are being finalized and it's going to be talked about tomorrow," said a second official.
The decision effectively shelves an earlier plan to shrink the size of the US force in Iraq to between 105,000 and 115,000 troops. There is now recognition that hostilities in Iraq are unlikely to subside after the handover of limited power on June 30.
Critics of US policy insist more troops are needed to pacify Iraq, and point to Washington's failure to commit the necessary forces as a major cause of the deteriorating security.
More than 750 US troops have now been killed in Iraq since the US-led invasion in March last year. April was the deadliest month for US forces since the start of the campaign.
Abizaid told reporters last week he did not favor big increases in US troops except to deal with the immediate security problems.
"Am I comfortable with where we are now? Militarily, yes," Abizaid said.
"If the situation were to move into less secure circumstances than are currently visible in the country, I would go to the secretary and ask for more forces, and General Sanchez agrees with me on that. But I don't see a need to do that now," he said, referring to General Ricardo Sanchez, his commander in Iraq.
US commanders had hoped to shift responsibility for security to Iraqi police and civil defense forces, easing the pressure on stretched US forces.
But many Iraqi units collapsed when riots and fighting erupted last month, as Sunnis and Shiites rallied against US forces in opposition to the year-old occupation.
Abizaid acknowledged that Iraqi security forces will have to be re-trained and re-equipped, and may not be ready to go before February of next year.
US troops also have had to fill a hole left by the withdrawal of Spanish and Latin American troops from the area around Najaf, where followers of radical Shiite cleric Moqtada Sadr have been attacking coalition forces.
Abizaid expressed hope that international troops, particularly from Muslim countries like Pakistan, Morocco and Tunisia, could be recruited to serve in Iraq after June 30.
But other senior US officials have said neither NATO allies nor other countries are likely to provide more troops so long as fighting continues in Iraq.