The chief of the US Federal Aviation Administration is set to detail on Monday progress on the Boeing 737 MAX aircraft to international air regulators who are divided about returning the grounded jet to flight after two fatal crashes.

The closed-door briefing, on the eve of a United Nations aviation assembly in Montreal, will put representatives from about 50 nations in the same room, to swap concerns about Boeing Co's proposed software fixes and new pilot training.

New FAA administrator Steve Dickson downplayed the chances of a consensus breakthrough, telling Reuters last week that the meeting was more "to provide regulators with the latest information."

The representatives are drawn from countries with airlines that fly that MAX and those that will have flights of the aircraft landing, he said.

Boeing's best-selling jet was grounded globally in March, days after the crash of an Ethiopian Airlines flight that followed a similar Lion Air disaster in Indonesia in October. A total of 346 people died in the two crashes.

The US manufacturer has spent months working to update critical flight control software at the center of both crashes, in hopes of winning FAA approval for the planes to fly again in the United States between October and December.

Airlines have urged regulators to coordinate on the software changes in a bid to avoid damaging splits over safety, but some countries have already vowed to run their own independent validation studies before restoring flights.

The European Union Aviation Safety Agency (EASA), which is doing its own examination of the 737 MAX design, said recently there was still no appropriate response to issues with the integrity of the aircraft's angle of attack system.

In both crashes, erroneous data to one of the angle of attack sensors led to the activation of an automated system that repeatedly pushed down the plane's nose.

Paul Njoroge, who lost his wife, three children and mother-in-law in the Ethiopian crash, and Chris Moore, who lost his daughter, plan to hold pictures of victims outside the meeting.

The FAA turned down their request to attend the briefing but Dickson agreed to meet the two men beforehand, said Nadia Milleron, whose daughter, Samya Stumo, also died in the Ethiopian crash.

The FAA is still reviewing the software update and awaiting a document longer than 500 pages from Boeing detailing the flight control system's architecture and changes, Dickson said last week.

The agency also wants input from international regulators EASA, Canada and Brazil before it conducts the certification test flight, a key step before final approval.

The 737 MAX grounding is not on the agenda of the assembly of the International Civil Aviation Organization, to run from September 24 to October 4, but regulators will be anxious to avoid divisions over actions needed to restore the jet to service.