In a letter sent to leaders of the House and Senate transportation committees last week, IATA and other organizations in the travel and shipping industries argued that the "flag of convenience" clause would violate the aviation agreement between the U.S. and the European Union, under which airlines are able to fly unfettered across the Atlantic.

Among the signatories of the letter are JetBlue, Atlas Air, UPS and FedEx. But Delta, United and American, which opposed the Norwegian air permit, are among the carriers that didn't sign on. Airline trade group Airlines for America is silent on the proposal.