US defence secretary Robert Gates is in the Iraqi capital, Baghdad, on a previously unannounced visit.
He is expected to oversee a ceremony at which Lt-Gen Ray Odierno will take over command of US troops in Iraq.
Gen Odierno will replace Gen David Petraeus, who is moving to a new job in charge of US forces in the Middle East and Horn of Africa.
Last week, Mr Gates said the Iraq war had reached its end-game, but the US should be wary about troop withdrawals.
He is expected to hold talks with Iraqi officials, in what is his second trip in eight months and his eighth since becoming Pentagon chief in December 2006.
Washington and Baghdad are negotiating a controversial security pact that will govern the US troop presence in Iraq when a UN mandate expires at the end of 2008.
“The challenge, I think, for Gen Odierno is: How do we work with the Iraqis to preserve the gains that have already been achieved, expand upon them, even as the numbers of US forces are shrinking,” Mr Gates told reporters on his flight to Baghdad.
He added that Iraqis must move forward with reconciliation between Shia Muslims, Sunni Arabs and Kurds
“There’s still people who would like to see this fail and the important thing will be to keep the pressure on all of them,” he said.
Violence has decreased in Iraq under Gen Petraeus’s strategies, which included a surge of US troop numbers to implement crackdowns on insurgents.
Mr Gates praised Gen Petraeus as “the hero of the hour” but said those working for him who put his plans into action were also “heroes”.
Correspondents say he was aided by factors such as the decision of former Sunni insurgents to turn against al-Qaeda and a ceasefire by the Mehdi Army militia.