Checks are to be carried out on a second Boeing aircraft model following the blowout of an unused door on one of its planes earlier this month.
The US Federal Aviation Administration grounded more than 170 of the 737 Max 9 fleet after a cabin panel broke away thousands of feet above the ground.
On Sunday, the agency said airlines should also inspect older 737-900ER models, which use the same door design.
The FAA described the move as an "added layer of safety".
It said there had been no reported issues with the 737-900ER, but that it uses the same style of panel to "plug" an unused door as the plane involved in the terrifying 5 January incident.
An Alaska Airlines flight en route to California from Portland, Oregon was forced to make an emergency landing after the panel came away, leaving a gaping hole in the side of the aircraft.
The incident prompted the FAA to ground all 737 Max 9s featuring that style of panel and sent Boeing's share price tumbling.
The agency is investigating the firm's manufacturing practices and production lines, including those linked to subcontractor Spirit AeroSystems, which provided the panel.
Earlier this week, the FAA said it had carried out inspections on 40 of the grounded planes but did not say when they would be able to fly again.
In a statement on Sunday, the agency said: "The safety of the flying public, not speed, will determine the timeline for returning these aircraft to service."
Boeing has said it will increase the quality of inspections in its manufacturing process in wake of the incident.
The 737-900ER models have carried out 11 million hours of operations without similar incident to the newer 737 Max 9s.
The FAA did not order the older model to be grounded while the visual inspections are carried out by operators.
