US defence contractor Lockheed Martin says it has thwarted a significant cyber-attack.
Few details were available, but Lockheed said its security team had detected the threat quickly and ensured that none of its programmes had been compromised.
The Pentagon said it is working to establish the extent of the breach.
Lockheed makes fighter jets, warships and multi-billion dollar weapons systems sold worldwide.
Lt Col April Cunningham, speaking for the US defence department, said the impact on the Pentagon was "minimal and we don't expect any adverse effect".
Lockheed Martin said in a statement that it detected the attack on 21 May "almost immediately" and took counter-measures.
As a result, the company said, "our systems remain secure; no customer, program or employee personal data has been compromised".
But they are still working to restore employee access several days after the attack took place.
Lockheed Martin is the world's biggest aerospace company and makes F-16, F-22 and F-35 fighter jets as well as warships.