Boeing 737 MAX airplanes are seen parked on Boeing property along the Duwamish River near Boeing Field on August 13, 2019 in Seattle, Washington.

"It will not improve the trust of the general public in the system" if countries have their own plans for the planes' return, Alexandre de Juniac, director general of the International Air Transport Association, said in a press briefing Tuesday.

The 737 Max, Boeing's best-selling plane, has been grounded worldwide since mid-March after two fatal crashes — one in Indonesia in October and another in Ethiopia in March — within five months of one another killed a total of 346 people

Aviation regulators around the world should come to an agreement on when the Boeing 737 Max can return to service, or risk hurting public trust with a piecemeal approach, the head of an airline trade group warned Tuesday.

The Federal Aviation Administration's new chief Steve Dickson on Monday briefed his international counterparts on the certification process of the 737 Max ahead of a United Nations aviation meeting in Montreal this week. He called for improvements to aircraft design and production standards.

The agency said it would be up to individual countries to decide when the planes can fly again and Boeing's CEO earlier this month said that the planes could return to service in phases.

The grounding, now in its seventh month, has forced airlines to cancel thousands of flights during the peak summer travel season and through the The FAA has said it has no set time frame to return the planes to service.

Investigators of both air disasters implicated flight-control software that was triggered in error, repeatedly pushing the nose of the planes down.

IATA's de Juniac stopped short of saying pilot training should be uniform around the world, amid questions over how prepared pilots are to manage ever-more complex aircraft.

"I'm not sure it's not necessary to have the same training," he said, adding that the U.N. aviation body should discuss it.