SEOUL—North Korean hackers stole wing designs for a U.S. jet fighter and photos of parts of spy planes from a South Korean company, according to authorities in Seoul, the latest in a series of alleged cyberattacks by Pyongyang.

More than 40,000 documents related to the defense industry were stolen in attacks on two companies that began in 2014 and were discovered earlier this year, the Korean National Police Agency said Monday.

One of the companies, Korean Air Lines Co., said it was told by police its systems had been breached. Among the documents stolen were wing designs for an F-15 jet fighter and photos of parts of unmanned spy planes, a spokesman for the company said. Korean Air makes aircraft parts for South Korea’s military, which flies F-15 jets.

A South Korean military official said the leak wasn’t of sensitive information, such as F-15 engines or electronic systems. “The leak will likely have a negligible impact on national security,” the official said.

The police said that by detecting the breach they prevented what appeared to be the start of a larger-scale cyberattack.