The U.S. Federal Aviation Administration (FAA) said on Tuesday it was revising its order requiring checks for structural cracks in Boeing 737 NG planes to cover a larger area after the company said additional cracks had been found.

The FAA first said in September it was ordering checks for cracks on what is known as the "pickle fork," a part that attaches the plane's fuselage, or body, to the wing structure. Checks were required within seven days on planes with heavy usage.

Boeing said on Tuesday after completing a round of inspections for cracking on some 737 NGs with a large number of take-offs and landings, one airplane was found to have a small crack in an adjacent location.

"Boeing has asked those operators to also inspect the adjacent area to ensure any potential issue is identified and repaired," the company said in a statement.

"The additional inspections are already underway. We regret the impact to our customers and have a repair plan in place to address any findings."

To date, less than 5% of the 1,200 airplanes that have undergone the initial inspection were found to have cracks, Boeing said. The secondary issue has been discovered on three in-service airplanes and one airplane that was undergoing maintenance.

The FAA said in a notice it was now expanding the inspection to the area around eight fasteners. Southwest Airlines Co found cracks in three airplanes after the initial inspections and has taken them out of service pending repairs.

Southwest said after it completed required inspections it decided to continue "inspections until every NG aircraft in our fleet has been checked."

The FAA directive requires airplanes with 30,000 or more cycles be re-inspected within 60 days, Boeing said. Airplanes with 22,600 to 30,000 cycles will need inspections of the full area within 1,000 cycles. A cycle is generally one take-off and landing.

Last week, Lion Air found structural cracks on two 737 NG planes with fewer flights than an FAA threshold for checks. The Lion Air jets with cracks had fewer than 22,000 cycles and are now grounded for repairs, a spokesman for the airline said.

Brazil's Gol Linhas Aereas Inteligentes SA, Australia's Qantas Airways and Korean Air Lines Co Ltd are among the other airlines that have grounded planes after discovering cracks.

First delivered in 1997, the NG is the third generation of the 737, preceding the grounded MAX which is unaffected by cracking.