The aircraft has been grounded since March 2019, following two fatal crashes that killed 346 people.
Federal Aviation Administration (FAA) chief Steve Dickson said his agency “has no set timeframe” to complete its review.
He issued the statement following a Reuters report that he had told airlines that Boeing’s public timeline was “very conservative” and that re-certification could come before July.
The news sent shares in Boeing up 2% on Friday.
The firm has been in crisis since the crashes, which occurred within five months of each other – first in Indonesia in October 2018 and then in Ethiopia last March.
It is facing multiple investigations amid accusations that it sacrificed safety as it rushed to get its jets to customers.
The FAA has also been accused of failing its oversight duty, due to a too-cosy relationship with Boeing.
The grounding of the 737 Max, which had been Boeing’s best-selling plane, is estimated to have cost Boeing more than $9bn already.
Last month, Boeing said it would halt production – a move that analysts say could shave 0.5% off US economic growth in the first three months of the year.
Suppliers have announced job cuts and airlines are grappling with schedule shifts. Many of the airlines have said they do not expect the 737 planes back in service before at least June.