Delta and American Express are extending their exclusive partnership that includes Delta SkyMiles Credit Cards, the companies said Tuesday.

The agreement, which now runs through 2029, will allow the companies to build on a partnership that has worked well for both.

"Enhancing customer loyalty is at the heart of our business. Our partnership with American Express provides a competitive advantage as we deliver substantial value to our customers and owners," Delta CEO Ed Bastian said in a press release announcing the extension.

The agreement focuses on the SkyMiles credit cards offered through American Express. Members holding Delta Reserve and Platinum versions of those cards come with perks that have become crucial to winning over travelers, including complimentary access to Delta Sky Clubs.

Stephen J. Squeri, Chairman and CEO of American Express, said the partnership added more than a million new SkyMiles Credit Card members last year.

"We are thrilled to extend our agreement in a way that combines so many of American Express' unique assets and capabilities with Delta's large and engaged customer base to drive growth for both companies," he said in a release from both firms.