He told reporters he did not expect significant results until the summer and fall, for the first time putting a timeframe around the new plan that was announced Wednesday by U.S. President George W. 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," Casey told a news conference.
U.S. Ambassador Zalmay Khalilzad, who joined Casey at the podium, declared that there were no deadlines being imposed on the Iraqis to take action against gunmen, but 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," 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 U.S. envoy said.
Both Casey and Khalilzad repeatedly said they had assurances that no areas of Baghdad would be allowed to serve as sanctuaries for gunmen, a direct reference to past interference on the part of Prime Minister Nouri al-Maliki to protect the Mahdi Army militia of radical Shiite cleric Muqtada al-Sadr, one of the Iraqi leader's key political backers.
Casey also said the security drive might take months to show results.
"I think you'll see a gradual evolution over the next two to three months, and then you'll see things continue to get better up through the summer and fall. It'll take some time," Casey said, standing at the side of U.S. Ambassador Zalmay Khalilzad.
Casey said he believed success for this attempt — the third in less than a year — to curb sectarian violence in Baghdad could succeed because the country's political leaders voiced a deeper commitment and had vowed not to protect any armed group, as it has in the past.
"We have been here before...but what's different is a stronger level of political commitment from the government and a better organization on the Iraqi side to help their own plan succeed. And I will tell you yes, there are still difficulties with the Iraqi security forces. That has been a challenge," said Casey, who will cede command of U.S. forces to Gen. David Petraeus.
Petraeus has served two 12-months stints in Iraq.
The general spoke as Baghdad saw a muted reaction Monday to the of Barzan Ibrahim, Saddam Hussein's half brother and former intelligence chief, and Awad Hamed al-Bandar, former head of Iraq's Revolutionary Court.
The two had been found guilty along with Saddam in the killing of 148 Shiite Muslims following a 1982 assassination attempt on the former leader in the town of Dujail north of Baghdad.
Khalilzad also is leaving his post for the United Nations, where he will become the U.S. envoy if confirmed by the Senate. He is being replaced by Ryan Crocker, current ambassador to Pakistan.
Casey also asserted that al-Maliki had approved the insertion of 21,500 new American troops into Iraq and would be consulted on all future U.S. troop additions or withdrawals.
The top U.S. commander said recent U.S. detentions of Iranians in Iraq were justified because the men taken captive were working for the Iranian government to supply weapons and other aid to insurgents and other anti-U.S. and Iraqi forces.
Five Iranians were taken in a raid on an Iranian facility in the northern Iraqi city of Irbil on Thursday
"These gentlemen were not diplomats that we're holding, and all (of them) by discussion that I've had with people ... they are Revolution Guards representative in a Kurdish area in northern Iraq. ...
"These folks that we have captured are foreign intelligence agents in this country working with Iraqis to destabilize Iraq and target coalition forces that are here at Iraq's request. I don't think there is any disagreement on that," Casey said.