Yahoo Inc. said Thursday it is investigating reports of a security breach that may have exposed nearly half a million users' email addresses and passwords.

The company said it was looking into "claims of a compromise of Yahoo! user IDs" but did not disclose the size of the reported breach or how it may have happened.

Yahoo's head of U.K. consumer public relations, Caroline MacLeod-Smith, said she couldn't immediately provide any more detail on the breach as the company was still investigating it.

Technology news websites, including CNET, Ars Technica and Mashable, cited hackers calling themselves the D33D Company as claiming responsibility for the attack, adding that data posted to the group's website carried more than 453,000 login credentials from an unidentified Yahoo subdomain.

The little-known group was quoted as saying that they had stolen the passwords using an SQL injection — the name given to a commonly used attack in which hackers use rogue commands to extract data from vulnerable websites.

"We hope that the parties responsible for managing the security of this subdomain will take this as a wake-up call," the group was quoted as saying.

A Ukraine-registered website associated with D33D Company appeared to be unreachable Thursday; an email address and a phone number attributed to the site's registrant appeared to be invalid.

The Yahoo breach follows a similar one last month in which about eight million passwords belonging to users of LinkedIn, the music streaming site Last.fm and the online dating site eHarmony were leaked to a password-cracking forum.