US defence firm Lockheed Martin says it has come under a significant cyber-attack, which took place last week.
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.
Councillor Ahmed Khan claims his human rights have been breached
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An English council has taken Twitter to court in the US in a bid to discover the identity of a blogger behind allegedly libellous statements.
South Tyneside Council went to court in California after three councillors and an official complained they were libelled in a blog called "Mr Monkey".
Twitter said it could not comment on individual court requests.
But the councillor at the centre of the row said Twitter had already handed over his account details.
Media law experts suggest the case may prompt more UK citizens to take action in the US, where Twitter is based.
Independent South Shields councillor Ahmed Khan is suspected of being the author of the blog, which has made a series of unfounded allegations against council leaders.
Mr Khan, who denies being the author, said he was told by Twitter in May that his account details had been disclosed after a subpoena was lodged with the Californian court.
He said: "I don't fully understand it but it all relates to my Twitter account and it not only breaches my human rights, but it potentially breaches the human rights of anyone who has ever sent me a message on Twitter.
Is this a landmark moment for free speech online, with Twitter handing over confidential details of a user for the first time?
Probably not. Twitter - like other major American web firms - complies with US court orders and requests from law enforcement agencies.
We know, for instance, that the US government sought to obtain details from Twitter of people connected with Wikileaks.
That case is still under way, because the targets were notified by Twitter and chose to fight. The South Tyneside councillor decided not to go to court and details of his account were handed over back in April.
But it seems unlikely that this was a first, for Twitter or the wider web. Google, for instance, issues an annual transparency report which reveals that in just six months last year, it received more than 4,000 requests for user data from US agencies and more than 1,300 from Britain.
So what are the implications for the Ryan Giggs case? Again, it's not clear this makes a difference.
Whereas the South Tyneside case involved a defamation suit brought in a US court, the footballer's lawyers are trying to get a UK court order imposed on a US company. Which is, as someone close to the case put it, a very complex business.
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"This is Orwellian. It is like something out of 1984."
He admitted being a critic of some council policies, adding: "People who had the courage to come forward and expose possible wrongdoing within South Tyneside Council will not now do so.
"I also think that constituents who have used Twitter to engage with me, to air any problems or concerns that they have, will also think twice before doing that."
The Mr Monkey blog has made a number accusations against the council's Labour leader Iain Malcolm, as well as David Potts, the former Conservative leader who now serves as an Independent councillor, Labour councillor Anne Walsh and Rick O'Farrell, the council's head of enterprise and regeneration.
They are all named on papers delivered by the council's lawyers to the Superior Court of California.
A spokesman for South Tyneside Council said: "This legal action was initiated by the council's previous chief executive and has continued with the full support of the council's current chief executive.
"The council has a duty of care to protect its employees and as this blog contains damaging claims about council officers, legal action is being taken to identify those responsible."
He said he had no knowledge of councillors attending court hearings in the US or whether Twitter had as yet handed over any confidential information.
'Seedy little blog'
A spokesman for Twitter said: "We cannot comment on any specific order or request.
"As noted in our law enforcement guidelines, it is our policy to notify our users before disclosure of account information."
Lawyers challenged Twitter in the High Court in London to reveal the identities of its users who violated a super-injunction involving Manchester United footballer Ryan Giggs.
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Media lawyer Mark Stephens says he is unaware of anyone from the UK taking action like this before
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MP John Hemming named the star in Parliament as the footballer who had used a super-injunction to hide an alleged affair, after Mr Giggs' name had been widely aired on Twitter.
Media lawyer Mark Stephens, who represented Wikileaks founder Julian Assange, said: "I am unaware of any other occasion where somebody from this country has actually gone to America and launched proceedings in a Californian court to force Twitter to release the identities of individuals.
"The implications are that people who have had their name released can actually now go to California and begin proceedings.
"Local authorities cannot sue for libel and, if individual councillors have been defamed, they should take proceedings at their own cost."
Mr Potts said: "This is a deeply tawdry, perverted and seedy little blog that has been in existence for quite a while.
"It's no longer active, as I understand, but the information is still on the internet for all to see.
"This was a blog that didn't just affect councillors, it also affected council officers.
"We have a duty of care, as any employer does whether public or private, to defend not only our commercial interests, but also the interests of our employees."
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