A computer hacker has gained access to more than 5 million Visa and MasterCard credit card accounts in the US.

Your Credit Cards are not Safe

The two companies said on Tuesday that none of the information obtained, which would include credit card numbers, was used in a fraudulent way.

But a UK-based business crime expert warned account holders could still be at risk if their cards were not reissued.

Visa and MasterCard said the hacker breached the security system of a company that processes credit card transactions on behalf of merchants.

Both Visa and MasterCard operate zero liability policies, which protect card holders from having to pay for any unauthorized or fraudulent charges.

More than 560 million Visa and MasterCard cards circulate in the US. MasterCard said it informed its members two weeks ago that more than 2 million accounts had been hacked into. Visa said about 3.4 million of its accounts were accesses by the computer hacker.

Both companies said they immediately alerted the affected banks that issued the cards.

Visa’s fraud team immediately notified all affected card issuing financial institutions, and is working with the third-party payment card processor to protect against the threat of a future intrusion.

The firms are also working with US law enforcement officials.