Civil aviation regulators from around the world have failed to make a determination on when Boeing's popular 737 MAX aircraft can return to the skies after being grounded following two deadly crashes.
"The only timetable is to make sure the aircraft is safe to fly," Daniel Elwell, acting head of the US Federal Aviation Administration, said at the conclusion of a meeting in Texas.
There was "enthusiastic agreement to continue the dialogue," he said, but acknowledged that "each country has to make its own decision."
"We can't be driven by some arbitrary timeline," Mr Elwell said.
Until the 737 MAX crashes in Ethiopia in March and Indonesia in October which left a combined 346 people dead, common practice was that air regulators would follow the assessment of the agency overseeing the model, in this case the FAA.
Earlier this week, Elwell threw cold water on hopes of a speedy resolution, after revealing Boeing had held off submitting a proposed software fix for review after his agency raised additional questions.
"Once we have addressed the information requests from the FAA, we will be ready to schedule a certification test flight and submit final certification documentation," Boeing said in a statement.
Investigators have focused on the MAX's anti-stall Maneuvering Characteristics Augmentation System in inquiries into the two deadly crashes.
Boeing last week said the MCAS update was ready for the certification process, and US airlines were hoping the planes could be back in the skies in time for part of the summer travel season.
But Elwell said the process could take one month, two months or longer. "It is all determined by what we find in our analysis of the application," he said on CNBC.
Once Boeing has submitted all documentation, the FAA will conduct a test flight and detailed analysis to evaluate the safety of the software.
Richard Aboulafia, an aviation analyst at Teal Group, said Boeing wants to avoid having to repeat the process.
"There's a lot at stake in terms of the first impression by the world's regulators," he told AFP.
US air carriers that operate the 737 MAX, including American Airlines, Southwest and United, have said they hope to have the planes flying again by mid-August at the latest.
The FAA's reputation has taken a beating since the March crash, and faced accusations of an overly cozy relationship with the aviation giant.
Other aviation authorities now appear less likely to follow the US agency.