Two of the largest U.S. airlines scrambled on Wednesday to rebook passengers whose flights were canceled when the Federal Aviation Administration followed its global counterparts in grounding Boeing’s 737 MAX.

American, which flew 145 million passengers in 2017, said it would make new travel arrangements for its customers “as quickly as possible” after learning of the FAA’s decision. The move followed mounting pressure after regulators worldwide cited similarities in the crash of a MAX flown by Ethiopian Airlines flight on Sunday with one six months earlier in Indonesia, both of which killed everyone aboard.