Hackers launched a "significant and tenacious" cyberattack on Lockheed Martin, a major U.S. defence contractor holding highly sensitive information, but its secrets remained safe, the company said Saturday.

Lockheed Martin, the U.S. Department of Homeland Security and the Pentagon confirmed that the contractor's information systems had come under attack. Lt.-Col. April Cunningham, speaking for the

Defence Department, said the impact on the Pentagon "is minimal and we don't expect any adverse effect."

Still, the concerted attempt to breach the contractor's systems underscored the risk to critical defence data. Chris Ortman, a Homeland Security spokesman, said his agency and the Pentagon were working with the company to determine the breadth of the attack and "provide recommendations to mitigate further risk."

Lockheed Martin said in a statement that it detected the May 21 attack "almost immediately" and took countermeasures. As a result, "our systems remain secure; no customer, program or employee personal data has been compromised." The company's security team is still working to restore employee access to the targeted network. Neither Lockheed Martin nor the federal agencies revealed specifics of the attack.

Lockheed is the maker of the F-16, F-22 and F-35 fighter jets as well as warships and other multibillion-dollar arms systems sold worldwide.

Canada is planning to buy 65 F-35s from Lockheed Martin through the Joint Strike Fighter project, a U.S.-led multinational purchase program that began in 1996. The stealth fighters would replace this country's aging CF-18 fighter jet fleet, but the purchase has been mired in controversy over its cost.