Chicago-based United Airlines is sounding the alarm over Alaska Airlines ’ planned acquisition of Hawaiian Airlines . Executives at the carrier raised concerns earlier this week, highlighting its current agreements with Hawaiian. It comes as the Department of Transportation (DOT) is reviewing the proposed merger following the Justice Department’s regulatory review period expiring earlier this month.

Alaska and Hawaiian still have not been given the green light to merge, but some believe they are a step closer. Sending a letter United’s Chief Legal Officer, Robert Rivkin, spoke about the concerns with Brian Stansbury, the DOT deputy general counsel, on Tuesday, according to Skift . However, the memo reportedly did not go into much detail about United’s unease with the merger.

The carrier’s partnership with Hawaiian includes interline, codeshare, pricing, strategy, and loyalty agreements. Alaska has maintained that the merger will provide greater choice and flexibility for passengers. Unlike traditional acquisitions, the airline plans to continue operating both brands separately, except for the loyalty program.

Currently, Alaska has its MileagePlan program, while Hawaiian has HawaiianMiles. If the airlines receive approval to merge, the programs will be combined, although a name for the program has yet to be announced. Existing miles from both programs will be integrated into the new program.

“Once the combination is complete, Hawaiian Airlines’ existing customers.