"So far, so good"
The first group of extra troops, a brigade of the 82nd Airborne Division, has just arrived in Baghdad. Gates said it was too early to predict how Bush's plan for quelling the sectarian violence in the capital will work.
Four other brigades are to be sent to Iraq between now and May, assuming the Iraqis follow through on their commitment to bring three additional Iraqi army brigades into Baghdad and to allow raids against all fighters.
Asked how the Iraqi government was doing to meet its commitments, Casey said: "So far, so good."
Casey stressed that it was too early to say with confidence how long the US military will have to maintain a higher troop level in Baghdad and western Anbar province.
He said: "You're going to see some progress gradually over the next 60 to 90 days."
Casey is being replaced soon by David Petraeus, a lieutenant general, as the top US soldier in Iraq, although the timing is uncertain.
He has been nominated to become the next chief of staff of the army, but has not yet been confirmed in that job by the senate.
Casey said: "It will be late summer before we see the results that would cause us to make some decisions like that," referring to the prospect of reducing the overall size of the American force in Iraq, which stood at 132,000 troops at the time Bush announced he was sending reinforcements.
Gates in talks
Gates' visit to Iraq, his second since becoming defence secretary, was not announced in advance.
He immediately went into talks with US commanders and their allied counterparts amid the burgeoning war policy debate in the United States.
He also visited Basra, Iraq's southern port city, where he met senior British military commanders.
Britain, which has the largest troop contingent among the US allies, with about 7,000 soldiers in the Basra area, is planning to withdraw a large portion of them this year.
Gates said at the outset of his one week overseas trip that he realized the security situation in southern Iraq is different than in Baghdad, where the US is building up its troop strength.
From Basra, Gates and Casey took a cargo plane to the Tallil air base near the ancient city of Ur and about 15km from the southern city of Nasiriyah.
They met there with commanders from several coalition countries, including Australia, Poland, Romania and Denmark.