More bad news for Boeing's 737 line of aircraft: The U.S. Federal Aviation Administration said late Friday it will require operators of some 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.

The 737 NG, or Next Generation, was introduced in 1997 and is the third-generation version of the best-selling Boeing airplane. It includes the 737-600, -700, -800 and -900 models and is operated in Canada by WestJet and Air Transat but not Air Canada. There was no immediate indication that either airline's fleets were affected.

Its successor is the 737 MAX series, which was grounded in March after two fatal crashes in five months, but which is not affected by this issue, Boeing said.

The FAA said it would ask operators of the NG to "report their findings to the agency immediately" after completing inspections. Boeing said "over the coming days, we will work closely with our customers to implement a recommended inspection plan for certain airplanes in the fleet."

KOMO News reported on Friday the issue involved cracked "pickle forks" in some 737 NG jets. The pickle fork attaches the plane's fuselage, or body, to the wing structure and manages forces. A failure of the part in flight could pose a serious risk. KOMO said workers found a severely cracked pickle fork on a Boeing 737NG earlier this month.

Transport Canada had not issued an online civil aviation safety alert about the issue as of early Saturday morning.