Cyber hackers nearly two years ago breached a high-tech jet fighter program developed for the Pentagon by Lockheed Martin Corp., but classified information about the aircraft was not compromised, a senior defense official said Tuesday.

No details about the attacks were provided. The defense official spoke on condition of anonymity because of the sensitive nature of the issue.

In confirming the attack on Lockheed's F-35 Lightning II program — also known as the Joint Strike Fighter — the defense official said it is not clear who did it, or whether it was an attempt at corporate thievery or a hacker trying to harm the program. The Pentagon is expected to pay about $300 billion to buy nearly 2,500 of the F-35 jets for the Air Force, Navy and Marines.

The cyber spying revelations come as the White House is poised to release its review on the nation's cyber security. There have been increasingly frequent warnings that the nation's networks are at risk and repeatedly are being probed by foreign governments, criminals or other groups.

Lockheed officials issued a carefully worded statement saying that "to our knowledge there has never been any classified information breach" but that the company's systems are continually attacked, and there are measures in place to detect and stop the hacking.

The statement did not specifically deny a breach into unclassified information or less sensitive areas of the program. The cyber attacks were first reported by The Wall Street Journal.

In the case of the fighter jet program, the intruders were able to copy and siphon off several terabytes of data related to design and electronics systems, officials told the Journal, potentially making it easier to defend against the attack.

While the spies were able to download sizable amounts of data related to the jet fighter, they weren't able to access the most sensitive material, which is stored on computers not connected to the Internet, the Journal reported.

Pentagon spokesman Bryanms, or to have the ability to do so at a time of one's choosing."

His comments were not related to the F-35, according to administration officials. But Brenner has also warned that careless, laid-off or disaffected employees can often be the root of corporate cyber leaks. Foreign governments or groups, he said, plan computer attacks that take advantage of sloppy workers or bad network management practices.

In a series of recent speeches, Brenner has repeatedly raised the alarm that foreign governments and other groups are accessing government systems and installing malicious software.

"The Chinese are relentless and don't seem to care about getting caught. And we have seen Chinese network operations inside certain of our electricity grids. Do I worry about those grids, and about air traffic control systems, water supply systems, and so on? You bet I do," Brenner told an audience at the University of Texas at Austin.

• Continue reading this story in the Wall Street Journal.

• Click here for a list of recent cases of cyberespionage.

• Click here to visit FOXNews.com's Cybersecurity Center.