Boeing said on Saturday that it was working closely with regulators to make the necessary changes to grounded 737 MAX aircraft to ensure their safe return.

The model has been grounded since March following the second of two crashes which left a combined total of 346 people dead.

"We are interacting daily with the FAA (Federal Aviation Administration)... and also with regulators around the globe" over the return of the 737 MAX, CEO of Boeing Commercial Airplanes Stan Deal told a press conference in Dubai.

"We continue to work diligently around the changes necessary for the airplane," Deal said ahead of the Dubai Airshow, an event that opens on Sunday.

"We continue to make progress."

The FAA, which has been widely criticised for entrusting certification of important systems of the aircraft to Boeing, has promised a thorough review before recertification.

On Tuesday, the FAA said it expected the airplane to resume flying in January, delaying its return by one month.

But on Friday, United Airlines said it had pushed back its expected date for 737 MAX aircraft to return to service, following similar announcements by rivals Southwest and American Airlines.

The US air carrier now says it expects flights to resume on March 4, 2020, two months later than previously estimated.

Boeing said this week it hoped to get regulatory approval for a return to service before the end of this year but has delayed its estimate for the resumption of commercial flights until January, to allow for pilot training.

Deal said approvals from the FAA and other regulators around the world will help set the schedule for the air-plane's return.

He said Boeing is discussing compensation for the grounding with its customers, including low-cost flydubai, a key client for the 737 MAX.

The grounding has exceeded initial expectations as Boeing had to upgrade systems and faced questions from regulators and politicians over the plane.

The 737 MAX crisis is one of the most serious in Boeing's 103-year history, and has already cost the company tens of billions of dollars, amid multiple investigations by US authorities and complaints from victims' families.

Copyright Agence France-Presse, 2019