WASHINGTON (Reuters) - A senior U.S. senator is pressing Hyundai Motor Co. and its affiliate Kia Motors Corp. on their plans to compensate customers for inflated fuel-efficiency claims the two companies admitted earlier this month.

Hyundai and Kia have agreed in negotiations with the Environmental Protection Agency to reimburse customers for additional fuel costs.

Under the plan, customers who purchased one of 13 Kia or Hyundai models from the 2011 to 2013 model years will receive a debit card to reimburse them for the difference in fuel economy and an extra 15 percent will be added to the account to acknowledged the inconvenience.

"While I believe this is a positive step, I am concerned that many affected customers may not learn about the program or may find it burdensome to participate," Senate Commerce Committee Chairman Jay Rockefeller said in separate letters to the heads of Hyundai and Kia's U.S. divisions on Thursday.

He pressed the executives to explain by December 14 how their companies will "maximize the effectiveness and the accessibility" of the program and their plans to reach customers who might not initially take advantage of it.

(Reporting By Doug Palmer; Editing by Bill Trott)