Southwest Airlines Co., the largest operator of the grounded Boeing Co. 737 Max, said the discovery of a new safety issue with the jetliner is extending its absence "well beyond" what was hoped, forcing the carrier to reassess its schedule for the rest of the year.

The latest problem, "unfortunately, is going to delay the time-line for returning the Max to service," in early October, Chief Executive Officer Gary Kelly told employees Monday. "That's very disappointing."

Separately, American Airlines Group Inc. suspended a daily round trip between its hub at Dallas-Fort Worth and Oakland, California, because of the Max grounding.