Hackers have gained access to the accounts of some Yahoo Mail users, after a "co-ordinated effort", Yahoo's Jay Rossiter said on Thursday. 

Yahoo believes hackers gained access through a third-party database, and not through its own systems. It has reset passwords on users affecte, who will be prompted to change their password. Users may receive an email notification or an SMS text if they have a mobile number on their account.

"We regret this has happened and want to assure our users that we take the security of their data very seriously," Rositter said in a post on Yahoo's blog.

Following the hack, Yahoo says it has put in place additional measures to protect Yahoo's systems. The company, which is behind the second-largest email service, didn't reveal how many accounts were affected. 

Hacks have become more prevalent across the internet in recent months. Adobe, Evernote, and LivingSocial have all had their servers broken into and user data taken. Accounts at Yahoo getting compromised on Thursday could have stemmed from user data being taken from other services.