"Our message to them is: We are working hard to obtain Sgt. Bergdahl's release, to bring him back into U.S. hands," Lapan said.
Asked about the family's complaint that the U.S. government has not done enough, Lapan said: "It's perfectly understandable that parents whose son has been kept in captivity for several years now are frustrated. We certainly understand that. That's why we do everything thing we can to try to keep them updated, to the extent we can."
He added: "If they are angry and/or frustrated, that is certainly understandable. I would say that our leaders are frustrated as well."
Bowe Bergdahl disappeared June 30, 2009, while deployed with his U.S. Army unit. He's spent three birthdays in captivity.
To solicit support for further action, Bob Bergdahl plans to speak at an annual demonstration to recognize prisoners of war over Memorial Day weekend in Washington. The event, organized by the nonprofit POW support group Rolling Thunder, typically attracts more than 100,000 motorcyclists to the nation's capital.
"With all that's going on, we really need a diplomatic path to get this resolved," Bob Bergdahl said.
AP National Security Writer Robert Burns contributed to this report.
Copyright 2012 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.