(Recasts, adds details)

By Kristin Roberts

WASHINGTON, June 6 (Reuters) - A day after he fired the top two Air Force leaders, U.S. Defense Secretary Robert Gates is expected to pick Michael Donley, a senior Pentagon official, as Air Force secretary, a senior defense official said on Friday.

Gates could ask President George W. Bush to nominate Donley, director of the Pentagon's administration and management office, as early as Friday afternoon.

Donley, who has previously served in senior civilian positions in the Air Force, would replace Michael Wynne, who was fired on Thursday after a series of mistakes involving the handling of nuclear weapons and parts.

Gates also fired the Air Force's top general, Chief of Staff Michael Moseley.

The decision to remove the Air Force's top leaders followed an investigation into the mistaken shipment of nuclear missile fuses to Taiwan -- a misstep the U.S. military never caught on its own. The probe's results were highly critical of Air Force actions and its leaders' response.

That incident was revealed in March, just months after another embarrassing Air Force mistake involving nuclear weapons. An Air Force bomber wrongly armed with nuclear weapons flew across the United States in August.

Several more Air Force generals and other senior Air Force and Army officers were identified as at fault in the mistaken shipment of fuses to Taiwan.

Pentagon press secretary Geoff Morrell would not say how many senior officers were named in the report, but said there were "several generals" identified.

Morrell said the Air Force's new leadership team, once in place, will determine what punishment to give those officers.

Gates will visit Air Force bases on Monday and Tuesday to reiterate his concerns about how the service has performed its most sensitive mission -- securing nuclear weapons and parts.

"The purpose of the visit is to reinforce the fundamental message yesterday -- that there is no room for error when it comes to safeguarding our nuclear arsenal and its associated components, and the Air Force and the entire military has to do a much better job in that area," Morrell said.

Gates will likely discuss other issues that have strained relations between the Air Force and Pentagon, such as spending priorities. The Pentagon chief has criticized the Air Force and other branches of the U.S. armed forces for focusing too much on weapons needed in future wars rather than items troops need now in Iraq and Afghanistan.

He has called this "next-war-itis."

The top-of-the-range F-22 fighter jet, for example, has not flown a single mission in either war but remains a priority among some senior Air Force officers.

Donley served as assistant Air Force secretary for financial management from 1989 to 1993. After that he served as acting Air Force secretary for seven months. (Editing by David Alexander)

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