WASHINGTON — A foreign intelligence service stole 24,000 computer files in March from a defense contractor developing systems for the US military, a top Pentagon official said Thursday.

"This was significant," Deputy Defense Secretary William Lynn told reporters.

"I don't think it set us back in terms of the development of the system," Lynn said, but the cyber attack on the unidentified contractor had "compromised information."

Lynn said the attackers swiped "data related to systems that are being developed for the Department of Defense."

"It was large -- 24,000 files," he said.

"It was done, we think, by a foreign intelligence service. In other words a nation state was behind it," he said.

Lynn declined to identify any suspects. "We don't get into our understanding of exactly who it was," he said.

The Pentagon official said the attack was "just the latest in a series" and not the largest ever on a US defense contractor.

"We've been getting hit for the better half of five or six years in a serious way," Lynn said at an event during which he unveiled the Pentagon's strategy for cyberspace.