BAGHDAD, Iraq — There are no guarantees of overall success or quick results in the new American-Iraqi security drive in Baghdad, the American commander in Iraq said yesterday.

General George Casey told reporters he did not expect significant results until the summer and fall, for the first time putting a timeframe around the plan announced Wednesday by President Bush.

“As with any plan, there are no guarantees of success, and it’s not going to happen overnight but with sustained political support and the concentrated efforts on all sides I believe that this plan can work,” General Casey said.

Ambassador Zalmay Khalilzad, who joined General Casey at the podium, declared there were no deadlines being imposed on the Iraqis to take action against gunmen but that the pressure was a perception that existed because of the Shiite-dominated government’s failure in the past to weed out Shiite militia fighters.

“The statements that have been made about the urgency for action reflects this doubt or concern that exists in many places about whether the Iraqis or the Iraqi leadership will decide to do what’s necessary,” Mr. Khalilzad said.

“I’ve discussed these issues with the prime minister and he’s said they need to move, not because of what we say or what’s said in Washington, but he has said ‘we have to move to secure the capital city because of Iraqi interests,”‘ the American envoy said. Their responses mirrored what other American officials have said in defense of Mr. Bush’s plan against a wave of criticism given the failure of two previous drives to secure the capital.

General Casey and Mr. Khalilzad — both set to leave their jobs soon — showed little emotion and did not offer much new information in the face of probing questions from reporters.