U.S. agencies offered assistance to Lockheed Martin Corp., the world’s largest defense contractor, which suffered what it called a “tenacious” cyber attack on its computer networks on May 21.

The Department of Homeland Security “is aware of a cyber incident impacting” Lockheed and, together with the Department of Defense, has offered help in “determining the extent of the incident, performing analysis of available data in order to provide recommendations to mitigate further risk,” Chris Ortman, a spokesman for the agency, said in an e-mail Saturday.

The impact on the military “is minimal and we don’t expect any adverse effect,” Lieutenant Colonel April Cunningham, a spokeswoman, said in an e-mail.

Lockheed, based in Bethesda, Maryland, said it detected a “significant and tenacious attack” on its computer networks on May 21. It was found “almost immediately” and no employee, program or customer data was lost, the company said in a statement.

Lockheed uses a mobile security system produced by EMC Corp.’s RSA unit. RSA bolstered security for clients, including Lockheed, after a network breach in March resulted in the theft of RSA data, a person familiar with the process said yesterday.

The remediation involves replacing the SecurID tokens issued by RSA that often expire in three years, said the person, who wasn’t authorized to discuss the matter publicly. The company’s defense-contractor clients, which make missiles, aircraft and other weapons, also include Northrop Grumman Corp. and Raytheon Co. Dave Farmer, a spokesman for EMC, declined to comment.

Information Taken

EMC, based in Hopkinton, Massachusetts, reported that the cyber attack in March resulted in information being taken from its systems, including data related to RSA’s SecurID authentication products.

Users who need access their employer’s secure network from remote locations may use a SecurID type device, a memory stick- like unit that generates random numbers that must be used in combination with a personal identification number to gain entry.

“If intruders get the key, the seed that enables one-time passwords to be generated,” then they may have the capability to break into networks that depend on such systems to authenticate users, Alan Paller, director of research at the SANS Institute, a computer security training institution in Bethesda, Maryland, said in an interview. Paller said he couldn’t say if Lockheed’s networks were breached.

Randy Belote, a spokesman for Los Angeles-based Northrop Grumman and Jon Kasle, a spokesman for Raytheon of Waltham, Massachusetts, declined to comment.