World's largest defense contractor Lockheed Martin Corp., which became the victim of a sophisticated cyber attack last week, has quelled fears that its sensitive data have been compromised.

Lockheed, which makes fighter jets, warships and multi-billion dollar weapons systems and is Pentagon's No.1 supplier by sales, confirmed on Saturday that it discovered nine days ago that its information technology system had sustained a significant and tenacious cyber attack.

Though the attack was of a serious nature, Lockheed allayed fears that the data on its customers, programs or employees could have been compromised.

The Bethesda-based company said its cyber security team had detected the attack almost immediately and had taken aggressive actions to protect its computer network. It said it is working around the clock to restore employee access to the network, while maintaining the highest level of security.

Though it faces “constant threats…from adversaries around the world,” Lockheed said “our policies, procedures and vigilance mitigate the cyber threats to our business, and we remain confident in the integrity of our robust, multi-layered information systems security.” In 2009, Lockheed fell prey to a hack attack, which experts believe originated in China. The hackers stole sensitive data on the US Air Force’s Joint Strike Fighter jet program.

The company did not confirm a Reuters story, which said its SecurID electronic keys had been breached.

SecurID, which works through electronic tokens or other systems, changes the user’s password every 60 seconds in an attempt to defeat hackers.

However, if the hackers had made copies of the SecurID tokens, it would enable them to gain remote access to Lockheed's network.

The SecurID system is supplied by RSA, the security division of EMC Corporation. EMC did not comment on the matter.

The US Defense Department also said it does not see any adverse effect on its weapons program because of the breach as Pentagon does not rely heavily on RSA's product solutions.