The Boeing 737 MAX may remain grounded for much longer after the Federal Aviation Administration (FAA) on Tuesday said it will take adequate time in reviewing Boeing's changes.

FAA said it "plans to issue a Notice of Proposed Rulemaking (NPRM) for an Airworthiness Directive (AD)" for the aircraft, which comes more than a year after the planes were grounded.

The authority in a statement said the NPRM will provide 45 days for the public to comment on proposed design changes and crew procedures to mitigate the safety issues.

The grounding of the 737 MAX followed investigations into a problem with a flight handling system which is believed to be responsible for two deadly plane crashes, including one in Ethiopia which killed 157 people in March.

"The FAA will not speculate when the work will be completed. The agency continues to follow a deliberate process and will take the time it needs to thoroughly review Boeing's work. We will lift the grounding order only after FAA safety experts are satisfied that the aircraft meets certification standards," the authority said.

(With input from agencies)