The Pentagon and Lockheed Martin, the lead defense contractor for the new F-35 Joint Strike Fighter, suggested yesterday that cyber-attacks had not caused any serious security breaches in the Pentagon’s most expensive weapons program.

Still, defense and corporate officials said attacks on the Pentagon as well as the F-35 program are constant and former defense officials familiar with the program said some of the F-35’s less sensitive systems have been infiltrated by cyber-intruders.

“We know we are probed on this every day. We have very aggressive defensive systems. The more sensitive the information, the greater the safeguards are,” said Pentagon spokesman Bryan Whitman. He said he was not aware of any sensitive F-35 technology having been compromised by a cyber-attack.

The comments came in response to a Wall Street Journal story Monday reporting that cyber-attackers copied and siphoned off data related to design and electronics systems, “potentially making it easier to defend against the craft.”

The F-35 is the Pentagon’s most expensive, complex and ambitious aircraft program. According to program estimates, the total investment required in the F-35 exceeds $1 trillion — more than $300 billion to buy 2,456 aircraft and $760 billion to keep them flying beyond their expected life cycle.