The US Federal Aviation Administration has identified a new potential risk that Boeing must address on its grounded 737 MAX jets before the aircraft can return to service.

Key points: It remains unclear whether the issue with the 737 MAX was due to a software or microprocessor fault

It remains unclear whether the issue with the 737 MAX was due to a software or microprocessor fault A hardware fix would further delay the aircraft's return to service

A hardware fix would further delay the aircraft's return to service The 737 MAX fleet was grounded earlier this year after two fatal crashes in which 346 people were killed

The issue was discovered during a simulator test last week and it is not yet clear whether it can be addressed with a software upgrade or will require a more complex hardware fix, sources with knowledge of the matter told Reuters.

The FAA did not elaborate on the latest setback for Boeing, which has been working to get its best-selling airplane back in the air following a worldwide grounding in March, ordered in the wake of two deadly crashes within five months.

The new issue means Boeing will not conduct a certification test flight until July 8 in a best-case scenario, the sources said, but one had cautioned it could face further delays beyond that.

The FAA is expected to spend at least two to three weeks reviewing the results before deciding whether to return the plane to service.

Last month, FAA representatives told members of the aviation industry that approval of the 737 MAX jets could happen as early as late June.

The world's largest plane maker has been working on the upgrade for a stall-prevention system known as MCAS since a Lion Air crash in Indonesia in October, when pilots were believed to have lost a tug of war with software that repeatedly pushed the nose down.

A second deadly crash in March in Ethiopia also involved MCAS.

In total 346 people were killed.

"On the most recent issue, the FAA's process is designed to discover and highlight potential risks. The FAA recently found a potential risk that Boeing must mitigate," the FAA said.

"The FAA will lift the aircraft's prohibition order when we deem it is safe to do so."

Boeing said in a securities filing that the FAA had asked it to fix a specific flight condition not covered in software changes already unveiled.

The US aircraft manufacturer said it had agreed with the FAA's decision and request and was working to address the problem.

"Boeing will not offer the 737 MAX for certification by the FAA until we have satisfied all requirements for certification of the MAX and its safe return to service," Boeing wrote in the filing.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 54 seconds 54 s Boeing is due to deliver about 5,000 737 MAX aircraft.

Unclear if problem due software or microprocessor issue

Boeing's aircraft are being subjected to intense scrutiny and testing designed to catch flaws even after a years-long certification process.

Two people briefed on the matter told Reuters that during a simulator test last week an FAA test pilot was running scenarios seeking to intentionally activate the MCAS stall-prevention system.

During one activation it took an extended period to recover the stabiliser trim system that is used to control the aircraft, they said.

It was not clear if the situation that resulted in an uncommanded dive can be addressed with a software update or if it is a microprocessor issue that will require a hardware replacement.

A hardware fix could add new delays to the plane's return to service.

The FAA also said on Wednesday it continued "to evaluate Boeing's software modification to the MCAS and we are still developing necessary training requirements".

"We also are responding to recommendations received from the Technical Advisory Board," it said.

"The TAB is an independent review panel we have asked to review our work regarding 737 MAX return to service."

American Airlines Group and Southwest Airlines had earlier cancelled flights until early September as a result of the grounding.

On Wednesday, United Airlines said it also was removing 737 MAX flights from its schedule until September.

ABC/Reuters