Jobs portal Monster.com has released more details about the severity of the attack on its site.

It said confidential details of more than 1.3 million people, mainly Americans, were stolen by malicious hackers who carried out the attack.

It said that servers in the Ukraine and hijacked home computers were used to mount the attack.

Monster said the information was stolen to lend credibility to e-mails sent out as part of a phishing campaign.