Boeing has been asked to address further software changes to its 737 MAX, after a “potential risk” was discovered.

The Federal Aviation Administration (FAA) has asked Boeing to make further changes to its MCAS software, after discovering an issue that would account for “a potential source of uncommanded stabilizer motion”.

“During the FAA’s review of the 737 MAX software update and recent simulator sessions, the [FAA] identified an additional requirement that it has asked the company to address through the software changes that the company has been developing for the past eight months,” Boeing said in a statement.

“The FAA review and process for returning the 737 MAX to passenger service are designed to result in a thorough and comprehensive assessment. Boeing agrees with the FAA’s decision and request, and is working on the required software.”

The issue, which reportedly surfaced during simulator testing, concerns the ability of pilots to quickly reassert control of the plane if an automated flight handling system pushes the plane downward, a person familiar with the matter told The Australian.

The FAA said that it was not operating on a prescribed timeline for returning the Boeing 737 MAX to service and that it “will lift the aircraft’s prohibition order when we deem it is safe to do so.”

“We continue to evaluate Boeing’s software modification to the MCAS and we are still developing necessary training requirements,” the FAA said in a statement.

The announcement comes after predictions the jet could return to service this month, after its four-month international grounding following two fatal crashes in Indonesia and Ethiopia that resulted in the combined deaths of 346 people.