A Pentagon spokesman attributed the growth to overlapping unit rotations, but it came amid surging violence that so far this month has claimed the lives of 101 US troops and many more Iraqis.
"Several units are transitioning out as several are transitioning in," said Lieutenant Colonel Mark Ballesteros, who said that as of Monday the number of US troops in Iraq was 150,000.
A Pentagon official, who asked not to be named, said the US Army's 4th Infantry Division was near the end of its year-long rotation.
US commanders in the past have timed the overlap of troop rotations to increase the US military presence during Iraqi elections and other critical milestones in the political process.
The increase this time follows the Muslim holy month of Ramazan, which US military officials anticipated would bring higher levels of violence, and comes just ahead of the November 7 congressional elections in the United States.
Pentagon officials, echoing Vice President Dick Cheney, have expressed concern in recent days that Al-Qaeda is trying to step up the violence in Iraq to influence the outcome of the US vote.
Eric Ruff, the Pentagon press secretary, said he did not know why US troops levels were climbing.
"This is news to me," Ruff told reporters. "Talk to MNF-I (Multi-National Forces-Iraq). That's General Casey's decision."
The increase is noteworthy because US troop strength in Iraq is only 10,000 under the all-time high of about 160,000 reported in January after the Iraqi elections.
It had fallen to as low as 127,000 in June when US commanders still believed they could make troop cuts this year.
In late July, US Defence Secretary Donald Rumsfeld reversed directions and ordered a combat brigade to be extended beyond a year as part of build-up to quell burgeoning sectarian violence in Baghdad.
In mid-September, General John Abizaid, the top US commander in the Middle East, said more than 140,000 troops would be needed through the first six months of 2007 to check the violence.
Since then, US troop levels have oscillated between 142,000 to 147,000, the level it reached last week.
But the stepped-up operations have so far failed to bring the violence under control in Baghdad or stop its spread to other cities, prompting a review of military tactics.
General George Casey, the commander of US forces in Iraq, had said earlier this month that it was an "open question" whether bringing in more troops would have a lasting effect on the violence.
But he left the door open to it last week at a press conference in Baghdad.
"Now, do we need more troops to do that? Maybe," he said. "And as I've said all along, if we do, I will ask for the troops I need, both coalition and Iraqis."
Rumsfeld, however, insisted two days later that Casey's comments had been mischaracterized in press reports.
"Listen, we're in the political season," he said. "People are trying to take what he said and turn it in a way that it plays the way they'd like to see it play."

Copyright AFP (Agence France-Presse), 2006