WASHINGTON - In a further shake-up of US defense leaders, the Pentagon's second-ranking official said Thursday he intends to resign but has agreed to stay on the job until new Defense Secretary Leon Panetta chooses a successor.

Deputy Defense Secretary William J. Lynn III said that he told Panetta last Friday, on Panetta's first day as Pentagon chief, that he planned to resign for personal reasons.

"I thought this was a logical point for me to depart the Pentagon," Lynn said during an interview in his office.

He said he told Panetta that he would be best served by having a deputy who was willing to stay at least through President Barack Obama's first term, which ends in January 2013.

"I did not think I could commit for that type of timeline," he said.

Lynn said he was leaving for "personal, family reasons," and wanted to spend more time with his children. He said it had nothing to do with Obama's choice of CIA director Panetta to succeed Robert Gates. Lynn said he knew Panetta only slightly from periodic contact while both served in the Clinton administration.

"The secretary asked me to stay to ensure a smooth transition" to a new deputy, "and we think that's probably early fall," or late this year, he said. "I think they'll try to move pretty quickly."

Panetta's chief spokesman, Doug Wilson, said Panetta has begun a search for a deputy. Wilson said it was possible that a nomination could come by the end of the summer.

Wilson said no other senior Pentagon officials have indicated they intend to leave anytime soon.

The Obama administration is in the midst of a major rejiggering of its national security team, and more changes are coming.