WASHINGTON (Reuters) — The U.S. Federal Aviation Administration late on Friday said it would require operators of some Boeing (BA.N) 737 NG jetliners to conduct inspections for structural cracks and make repairs as needed following the discovery of cracks on a small number of planes.

The FAA said Boeing notified it of the issue “after it discovered the cracks while conducting modifications on a heavily used aircraft.” Subsequent inspections “uncovered similar cracks in a small number of additional planes.” Boeing said on Friday it has been in contact with 737 NG operators about a cracking issue, but added that “no in-service issues have been reported.”

Neither the FAA nor Boeing immediately said how many planes were impacted by the required inspections.