Washington Secretary of State Condoleezza Rice declined on Wednesday to rule out American forces still being needed in Iraq a decade from now.
Senators warned that the Bush administration must play it straight with the public or risk losing public support for the war.
Pushed by senators from both parties to define the limits of U.S. involvement in Iraq and the Middle East, Rice also declined to rule out the use of military force in Iran or Syria, although she said the administration prefers diplomacy.
"I don't think the president ever takes any of his options off the table concerning anything to do with military force," Rice said.
Rice appeared before the Senate Foreign Relations committee for only the second time since members gave her an unexpectedly tepid endorsement to replace Colin Powell in January, and she fielded pointed questions about U.S. intentions and commitment on Iraq from lawmakers who said they are hearing complaints at home.
"Our country is sick at heart at the spin and false expectations," Sen. Barbara Boxer, D-Calif., told Rice. "They want the truth and they deserve it."
Rice said that Iraq's police and Army forces were becoming better able to handle the country's security without U.S. help, and she repeated President Bush's warning that setting a timetable for withdrawal plays into terrorists' hands.
"The terrorists want us to get discouraged and quit," Rice said. "They believe we do not have the will to see this through."
Rice said the United States will follow a model that was successful in Afghanistan. Starting next month, she said, joint diplomatic-military groups - called Provincial Reconstruction Teams - will work alongside Iraqis as they train police, set up courts and help local governments establish essential services.
An AP-Ipsos poll this month found 61 percent of respondents disapprove of Bush's handling of Iraq while 32 percent said they approve. In August, 53 percent said the United States made a mistake by going to war while 43 percent said it was the right decision.
The figures represent a sharp drop-off from strong support for the war in the early going.