WASHINGTON (Reuters) - U.S. and European Union regulators said on Tuesday they were still reviewing changes Boeing Co made to 737 MAX software after two fatal crashes, a development that raised questions about how quickly the grounded aircraft can return to service.

FILE PHOTO: Employees walk by the end of a 737 Max aircraft at the Boeing factory in Renton, Washington, U.S., March 27, 2019. REUTERS/Lindsey Wasson/File Photo

The ongoing safety review means a key 737 MAX certification test flight is unlikely before November, two sources said. Boeing has repeatedly said it hopes to resume flights in the fourth quarter, which began on Oct. 1.

Regulators sought to play down talk of transatlantic divisions over safety changes to the 737 MAX, which was grounded worldwide in March after two crashes killed 346 people within five months.

The U.S. Federal Aviation Administration (FAA) said in a statement it has a “transparent and collaborative relationship” with other civil aviation authorities as it continues its 737 MAX safety review. Its European Union counterpart said it was in “continuous contact” with both the FAA and Boeing.

“We do not at this stage have any specific concerns resulting from that assessment that would mean that we could not agree to a coordinated return to service. We are in continuous contact with both the FAA and Boeing,” a European Union Aviation Safety Agency (EASA) spokeswoman said by email.

FAA Administrator Steve Dickson told Reuters in September the agency would need about a month following the yet-to-be scheduled certification test flight before the planes could return to service.

The FAA reiterated on Tuesday that it does not have a firm date for completing its review.

Among its changes, Being is addressing a flaw discovered in the software architecture of the 737 MAX flight-control system that involves using and receiving input from both flight control computers rather than one.

For decades, 737 models have used only one of the flight control computers for each flight, switching to the other computer on the following flight, Reuters reported in August.

FAA still needs to see Boeing’s “final system description” - a “500-ish page document that has the architecture of the flight control system and the changes that they have made,” Dickson said last month. It then will need to complete an “integrated system safety analysis” and conduct pilot workload management scenarios.

Boeing plans to revise the 737 MAX software to take input from both angle-of-attack sensors in the anti-stall system linked to the two deadly crashes and has added additional safeguards.

Separately, a $50 million Boeing assistance fund that announced Sept. 23 it would pay each family $144,500 in the two deadly crashes has already paid three of the five claims submitted to date, the fund’s co-administrator, Camille Biros, told Reuters on Monday.

Southwest Airlines Co is scheduling without the MAX until at least early January, pending regulatory approval for commercial flight. American Airlines Group and United Airlines have canceled MAX flights through part of December.