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 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 the CDK cyber outage entered third consecutive day, impacting some dealers' software management tools.

CDK, which provides software to car dealerships, experienced another cyber incident on Wednesday and proactively shut down most of its systems.

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 impact 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."

In its statement on Thursday, CDK did not disclose how many dealerships were affected. But according to its website, the company works with more than 15,000 retail locations across North America.

Reporting by Nathan Gomes in Bengaluru, additional reporting by Juveria Tabassum; Editing by Alan Barona and Pooja Desai