Gen Peter Schoomaker said the United States was in a tough fight with insurgents in Iraq but his plans did not mean it would necessarily need to maintain the present level of 15 combat brigades there for the next four years.
Including these 15 combat brigades of roughly 3,500-4,000 soldiers, the United States has about 141,000 troops in Iraq.
"We don't know what's going to happen but I'm telling you we're looking at our force and how we would continue this level two (troop) rotations beyond (now), so that's beyond 2010," he told reporters at the Pentagon.
"It's tough, there's no question about it, but I think we're doing well," Gen Schoomaker said.
"This is not a prediction that things are going poorly or better. It's just that I have to have enough ammo in the magazine that I can continue to shoot as long as they (commanders in Iraq) want us to shoot," he said.
US commanders had hoped to cut the number of troops in Iraq this year, more than three years after the invasion which toppled Saddam Hussein.
But Gen John Abizaid, the top US commander for the Middle East, said last month current levels would have to be sustained at least through next spring as U.S. troops battle the insurgency and sectarian violence.
More than 2,750 US troops and at least tens of thousands of Iraqi civilians have died in Iraq since the invasion in March 2003. At least 39 US soldiers have been killed in Iraq since the start of this month alone.
Gen Schoomaker said the US would not lose militarily in Iraq but ultimately it would be up to Iraqi authorities to solve the country's problems. The US role was to help prepare Iraqi forces for that challenge.