Defense Secretary Robert Gates, right, walks Thursday at an undisclosed location with Gen. John Abizaid, who is soon leaving his post as head of U.S. Central Command. Gates arrived unannounced today in Iraq.

TALLIL AIR BASE, Iraq — Gen. George Casey, the top American commander in Iraq, said today that some of the extra troops that President Bush ordered to Baghdad could begin leaving by late summer if conditions allow.

"I think it's probably going to be the summer, late summer, before you get to the point where people in Baghdad feel safe in their neighborhoods," Casey told reporters at a news conference with visiting Defense Secretary Robert Gates.

Asked how long the 21,500 extra U.S. troops are likely to be kept in Iraq, Casey replied, "I believe the projections are, late summer."

The first group of extra troops — a brigade of the 82nd Airborne Division — has just arrived in Baghdad, and 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 illegal militias.

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 U.S. military will have to maintain a higher troop level in Baghdad and western Anbar province. But he sounded an optimistic note.

"You're going to see some progress gradually over the next 60 to 90 days," he said.

Casey is being replaced soon by Lt. Gen. David Petraeus, although the timing is uncertain.