Global airlines have urged regulators to co-ordinate on software changes to the Boeing 737 MAX, thus avoiding damaging splits over safety seen when the aircraft was grounded in March.
The International Air Transport Association, whose 290 carriers account for 80 per cent of world flying, said trust in the certification system had been damaged by a wave of separate decisions to ground the jet, with the US last to act.
Airlines are worried further splits between regulators over safety could confuse passengers and cause disruption.
"Any rift between regulators is not in anyone's interest," director general Alexandre de Juniac told an IATA annual meeting in Seoul on Sunday.
Boeing's best-selling jet was grounded in March after two crashes over five months killed a total of 346 people.
The Federal Aviation Administration initially resisted the decisions led by China, then Europe and others, to ground the plane, but later followed suit.
Airline officials say any new bout of staggered decisions could cause problems in operations and code-sharing.
But the European Union's top transport official said it reserved the right to plot its own course. The European Aviation Safety Agency has emerged as one of the toughest regulators during the crisis.
"Certainly EASA will take a very close look at the results (of proposed design changes) and then make a decision and that message was very clearly passed," Transport Commissioner Violeta Bulc told Reuters.
"We always work together with other regulators and we certainly will take joint moves, but EASA will reserve the right to take an individual look at the results and then of course engage with the rest of the regulators."
Asked how long it would take to end the crisis, she said, "I hope as soon as possible, because we do need to restore order and trust and move on."
The FAA says it has no target but has indicated privately to other regulators that it aims to certify new software by end-June, after which it could take weeks to get planes flying.
However, Tim Clark, president of Emirates, one of the world's largest airlines, warned it could take six months to restore operations as other regulators re-examine a practice of taking their cue from the FAA on Boeing jets.
"That is why it is going to take time to get this aircraft back in the air. If it is in the air by Christmas I'll be surprised - my own view," he told reporters.
