"It's going to take more time than has been allotted for the process to take hold," the New York Democrat said during a brief stopover to visit US troops in Kuwait on Saturday.
"I don't think we should be setting artificial timelines as this is a very challenging undertaking and we need to work with our Iraqi counterparts and make sure that the steps that are being taken are going to work."
Washington's plan for handing power back to Iraqis has been criticised by the top cleric among Iraq's Shia majority for paying too little heed to Islam and including too little Iraqi involvement.
Clinton arrived from Iraq with fellow Democrat Senator Jack Reed, who said the United States had overextended itself in Iraq. He said a failure to get the Shias' backing for the handover plan could cause huge political problems in the future.
"We could look back and see the decision to attack Iraq was one that ended up being very, very costly"
"We could look back and see the decision to attack Iraq was one that ended up being very, very costly and peripheral to the real enemies that are trying to attack the United States."
"And its connection with the terrorist cells that attack the United States are tenuous at best," he added, casting doubt on one of the administration's justifications for the invasion.
He said the US forces in Iraq and Afghanistan had critical shortages of specialised troops such as military police, civil affairs and psychological operations troops.
Clinton (C) meets prominent Iraqi
women in Baghdad
"We are beginning to see the cracks now because as these forces rotate out, and many of these troops here are reserve
National Guard people who have that expertise, we don't have a second echelon of those types of troops," Reed said.
Clinton has ruled out a presidential bid next year, but Iraq is emerging as perhaps the greatest threat to Bush's re-election as American troops suffer casualties almost every day.
More troops needed
She said more troops, preferably an international force, were needed in both Iraq and Afghanistan.
"We are fighting an enemy which has a lot of impact by relatively small numbers and we've got to provide security
throughout large countries. That's not easy with the force
numbers that we have.
"In Iraq, I still think the administration should internationalise the military, political, civilian presence," she said.
"And that means to go to the United Nations, to go to NATO and to go to other willing allies and be willing to share the authority and power as well as the responsibility."