Sony said Saturday about 2,500 customers' names and partial addresses stolen by hackers had been discovered posted online as it struggled to recover from the biggest-ever Internet security break-in.

"The website was out-of-date and inactive when discovered as part of the continued attacks on Sony," US-based Sony Electronics said in an online statement.

The data belonged customers, mostly Americans, who had entered a 2001 product sweepstake, the company said.

The company "immediately took the website down, and we are aggressively removing any residual links to the list", it said, without giving further details on the site.

An attack by hackers last month resulted in the theft of names, email addresses and possibly credit card information from more than 100 million accounts using Sony's online platforms.

The company later said its Sony Online Entertainment platform was also hit.

Sony said it would offer US PlayStation Network and Qriocity users free membership for 12 months in a deal that would include a $1 million per user insurance policy against identity theft.

It added that it was working to make similar programmes available elsewhere.

The breach is a blow for Sony as it focuses on pushing content such as games and music through hardware platforms including game consoles, smartphones and tablet computers amid competition from Apple's iTunes and App store.

Sony in Japan also said Saturday that it is unlikely to meet its self-imposed deadline to restore the PlayStation Network and other online services.

Sony shut down the network and the Qriocity music streaming service on April 20 to investigate the hack and secure them from future attacks, and said last Sunday that it would begin restoring them within a week.

But it said Saturday: "The company is taking time to strengthen the protection and coding of data, in addition to building necessary surveillance functions for actualising higher security.

"The company will resume the services in stages in different regions after assuring the safety of customers," it added in a statement.