Lockheed Martin, maker of weapons and war planes including this F-16i, called the attack "significant" [GALLO/GETTY]
Lockheed Martin Corporation, the world's largest military contractor, has been attacked by hackers in what officials say is "significant and tanacious" cyber attack.
The world's biggest aerospace company and US government's top information technology provider said on Saturday it thwarted the cyber attack but refused to give further details.
The US government is said to be working in tandem with Lockheed Martin, based in Maryland, to ease fears of vulnerability.
The company, which also produces missiles, said no customer, programme employee personal data was compromised due to "almost immediate" protective action taken after the attack was detected on May 21.
The US defence department said in a statement late on Saturday night that it was working with Lockheed Martin to determine the scope of the attack.
The incident's impact on the department is "minimal and we don't expect any adverse effect," Air Force Lieutenant-Colonel April Cunningham said by email.
But both the US government and Lockheed Martin refused to comment on details of the attack, including where it could have originated.
Several cybersecurity experts with extensive government dealings said they were in the dark about the origin of the attack.
But some said that the attack shows the level of vulnerability of US security forces in the context of digital content.
"I think it tells us that DHS [the US Department of Homeland Security] doesn't know much about what's going on," said Anup Ghosh, a former senior scientist at the Pentagon's Defence Advanced Research Projects Agency who worked on securing military networks.
Ghosh, who now runs a software security company, said there had been a string of intrusions against defence contractors, security companies and US government labs since the start of 2011.
In 2009, hackers were reported to have accessed computers holding data on Lockheed's projected $380bn-plus F-35 fighter programme, the Pentagon's most expensive arms purchase.
William Lynn, deputy US secretary of defence, wrote in the journal Foreign Affairs last fall that more than 100 foreign intelligence organisations are trying to break into US networks.
Some already have the capacity to disrupt US information infrastructure, he wrote.
Lockheed Martin produces F-16, F-22 and F-35 fighter jets as well as warships and other multibillion-dollar arms systems sold worldwide.
In its Saturday statement, the company said its sales from 2010 from "continuing operations were $45.8bn".
Lockheed Martin has been the target of anti-war protests for decades for manufacturing widely-used weapons and war planes.