US Senator Hillary Clinton has said President George W Bush's administration is moving too fast in a planned transfer of power to the Iraqis by July.
"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.
"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 Shi'ite Muslim majority for paying too little heed to Islam and including too little Iraqi involvement.
Mrs 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 Shi'ites' 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 and peripheral to the real enemies that are trying to attack the United States."