BOSTON (Reuters) - The US Department Of Homeland Security warned retailers about a type of malicious software attacking point-of-sales systems, dubbed "Backoff", that it said is undetectable by most types of anti-virus software.

The agency released a 10-page advisory about the payment-card-stealing virus Backoff on Thursday, saying it has been observed in at least three forensic investigations into breaches of payment systems.

The US government has released reports on several types of malicious software that cybercriminals used to steal payment cards in the wake of last year's unprecedented breach on Target Corp, which resulted in the theft of some 40 million payment card numbers.