NEW YORK (Reuters) -United Airlines may not take delivery of the Boeing 737 MAX 10 until 2027 or 2028 because of uncertainty about when the plane will be approved for use, an executive for the carrier said Tuesday.

Boeing ( BA ) has faced significant delays in getting the smaller MAX 7 certified, which must happen before the U.S. Federal Aviation Administration can then certify the larger MAX 10.

United Chief Commercial Officer Andrew Nocella told reporters on the sidelines of an event that the airline is taking MAX 9 airplanes as it waits for the certification of the 10.

"In supply chain terms, we want to make sure we get our aircraft so we've committed to the MAX 9," Nocella said. "Until we know the MAX 10 is going to be delivered, we don't want to have an aircraft that doesn't arrive."

Nocella said United may not achieve its earlier goal of being one of the first delivery customers for the MAX 10.

"We want to see the aircraft certified before we make the firm commitment to convert our 9s to 10s."

Boeing ( BA ) did not immediately comment.

In 2017, United made a major order of the MAX 10, which is larger and can carry more passengers. The 737 is Boeing's ( BA ) top-selling airliner. The FAA has currently capped production of the MAX at 38 planes per month after a 2024 mid-air emergency.

The FAA must approve an engine de-icing fix for the MAX 7 before it can certify the plane after Boeing ( BA ) in January 2024 withdrew a waiver request.

Alaska Airlines told Reuters last year it did not expect to receive 737 MAX 10 airplanes until at least mid-2026.

Nocella said there is no end in sight to supply chain issues on engines and other parts.

The new 787-9 with updated interior "should have been announced six months ago" but for supply chain problems, he said, adding: "I don't have a crystal ball, I can't tell you when it's going to be fixed."