(Reuters) -A group of hackers claiming responsibility for a cyberattack on CDK's software systems has demanded millions of dollars in ransom to put an end to the hack, which affected auto dealers across the United States, Bloomberg News reported on Friday.

The group behind the attack is believed to be based in eastern Europe, according to the report, which cited people familiar with the matter.

The auto retail technology and software provider planned to pay the ransom, Bloomberg News reported.

CDK, which provides software to car dealerships, did not immediately respond to a Reuters request for comment.

U.S. auto retailers Sonic Automotive and Penske Automotive flagged a hit to their operations on Friday, as CDK experienced a third consecutive day of outage.

In an emailed statement, Ford said that there was an industrywide system outage for dealers who use CDK.

"However, many Ford and Lincoln customers are able to receive sales and service support due to alternative processes available to our dealers," Ford said.

Sonic said its dealerships were open and it was working to minimize the disruption. The company said it was unable to determine if the outage would affect its financial condition, but that it did have a "negative impact" on its operations.

Penske said its Premier Truck Group business, which sells new and used commercial trucks, also uses CDK's disrupted dealer management system.

"Premier Truck Group has implemented its business continuity response plans and continues to operate at all locations through manual or alternate processes," Penske said.

The commercial truck dealership business has lower unit volumes than Penske's automotive dealership business, which does not rely on CDK's software.

Separately, Kia America told Reuters it was working with affected dealers to reduce the impact of the outage and continue towards "business as usual".

CDK on its website says it works with more than 15,000 retail locations across North America.