The planes have been grounded worldwide since mid-March, and the news on Monday suggests that airlines could be forced to park their Max jets longer than they had expected. Airlines that fly the jets are deploying other planes to fill some flights while canceling others.

“We are aware that the resumption of service for our 737 Max aircraft may be further delayed, and our team will work with all customers impacted by any flight cancellations,” said Ross Feinstein, a spokesman for American Airlines.

American had been pointing toward a late-April return of its 24 Max 8s. Over the weekend, Southwest Airlines announced that its 34 Max 8s would be removed from the schedule through May instead of mid-April. United Airlines has idled its 14 Max 9s through June 5.