But the the official also said Wednesday classified information about the stealth F-35 Joint Strike Fighter was still "well protected," ABC News reported.
The main problem was information stolen from contractors could allow an adversary to speed up its own version of the airplane.
The F-35 was among many weapons targeted by cyberattacks, a Pentagon report said.
Before an appropriations subcommittee Wednesday, Frank Kendall, the defense undersecretary for acquisition, technology and logistics, told senators the theft of unclassified information was a "major problem for us," but added he was confident classified information about the plane was still well protected.
But he also said, "I'm not at all confident that our unclassified material is as well protected." He said some of the information about the F-35 was unclassified "because it's not as sensitive or important."
The U.S. Defense Department plans to buy as many as 2,450 of the warplanes for the Air Force, Navy and Marines. ABC said cost overruns and production delays have increased the cost of each aircraft to almost $137 million.
ABC said China and Russia were developing their own stealth aircraft.