NEW YORK: Shares of Boeing fell Thursday after US regulators identified a new issue in the Boeing 737 MAX that will likely slow the plane's return to service following two deadly crashes.

Near 1500 GMT, Boeing was down 2.6 percent at $365.18, the biggest loser in the Dow.

Late Wednesday, the Federal Aviation Administration said it would require Boeing to mitigate a new "potential risk" in the 737 MAX. The planes have been grounded since mid-March following two crashes that claimed 346 lives.

The issue, identified during simulator testing, concerns a problem with a microprocessor that impedes the ability of pilots to quickly reassert control of the plane after having activated the  Maneuvering Characteristics Augmentation System (MCAS), according to two people familiar with the issue.

Even before this latest issue surfaced, the outlook for getting the planes back in the air was uncertain, in part because the FAA would like other regulators to approve the plane's reentry soon after the US agency.

On Thursday, the International Air Transport Association, which represents 290 airlines, called on global regulators to coordinate to ensure the aircraft's safe return to service.

"Aviation is a globally integrated system that relies on global standards, including mutual recognition, trust, and reciprocity among safety regulators," IATA chief executive Alexandre de Juniac said in a statement.

"Aviation cannot function efficiently without this coordinated effort and restoring public confidence demands it."

IATA also called for "additional training requirements" for 737 MAX crews but did not take a position on whether simulator training should be required of pilots in order for the plane to be certified back into service.

Also Thursday, Southwest Airlines again pushed back its timeframe for returning the 737 MAX to service, saying it was awaiting guidance on the timing from Boeing and the FAA.

"We previously revised our flight schedule by removing the MAX through Sept 2 to offer reliability to our operation and stability for our customers during the busy summer travel months," the company said in a statement.

"With the timing of the MAX's return-to-service still uncertain, we are again revising our plans to remove the MAX from our schedule through Oct 1." —AFP