The Federal Aviation Administration said Wednesday that Boeing 737 Max planes which have been grounded since March may not fly again until December, according to Bloomberg. It echoes similar sentiments last week by a Boeing executive.

The timeline was indicated during a speech by FAA associate administrator for aviation safety Ali Bahrami. The comments came at a joint conference in Germany by the FAA and Europe air safety agency, EASA.

The aircraft was grounded worldwide in March after two deadly crashes in the span of five months. Boeing is working on a software fix for a flight-control system linked to the crashes.

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Sensors malfunctioned during a Lion Air flight in Indonesia in October and an Ethiopian Airlines flight from Addis Ababa in March, causing anti-stall software to push the planes' noses down. Pilots were unable to regain control, and both planes crashed. In all, 346 people were killed.

It is not clear whether either crash could have been prevented if the cockpit alert had been working.

Bahrami indicated there is a lot of pressure on the FAA to get the Max flying again, but added it will happen “when we believe it will be safe," Bloomberg reported.

Boeing Chief Executive Officer Dennis Muilenburg reportedly told CNBC last week he also expected the aircraft to return to service by the end of the year.

The loss of the jet has led to major disruptions for airlines who rely on it for service. Bloomberg reports American Airlines has taken it off its schedule through Sept. 3. Southwest and United were looking to resume service in August. Now, all those airlines may need to change their plans.

The Associated Press contributed to this report.