WASHINGTON (Reuters) - The FBI (news - web sites) warned on Thursday that organized hacker groups, especially from Russia and Ukraine, have targeted vulnerable U.S. computer systems, stealing credit card information and then attempting to extort money by offering security services to the victim company.

More than 40 victim companies in 20 states have been identified and notified in ongoing investigations, the FBI-based National Infrastructure Protection Center said. More than 1 million credit card numbers have been stolen so far.

The hackers specifically targeted computer systems associated with e-commerce or e-banking, the FBI said.

``The investigations have disclosed several organized hacker groups from Eastern Europe, specifically Russia and the Ukraine, that have penetrated U.S. e-commerce computer systems'' by exploiting vulnerabilities, the FBI said.

Once the hackers gain access, they download proprietary information, customer databases and credit card information, FBI officials said. The hackers then contact the company through facsimile, e-mail or telephone.

After telling the company about the theft of the information, the hackers make a veiled extortion threat by offering Internet security services to protect against other intrusions, they said.

If the company fails to get the services, the company is warned that hackers may post the credit card information and details about the compromise on the Internet, they said.

The hackers will increase the threats if the company does not cooperate in making payments or hiring the group for their security services, the officials said.

In some instances, the credit card information has been sold to organized crime groups.