WASHINGTON (Reuters) - The Federal Aviation Administration (FAA) said on Friday it seeks to impose a $3.9 million fine on Boeing Co, alleging it failed to prevent the installation of defective parts on about 130 737 NG airplanes.

FILE PHOTO: Boeing employees work on the tail of a Boeing 737 NG at the Boeing plant in Renton, Washington December 7, 2015. REUTERS/Matt Mills McKnight

The FAA alleged that Boeing “failed to adequately oversee its suppliers to ensure they complied with the company’s quality assurance system ... Boeing knowingly submitted aircraft for final FAA airworthiness certification after determining that the parts could not be used due to a failed strength test.”

In a statement on Friday, Boeing did not admit liability but said they were aware of the FAA’s concerns.

“We are working closely with our customers to take the appropriate corrective actions,” spokesman Charles Bickers said.

The company has 30 days to respond either by paying the fine or challenging it; Boeing said it would review the penalty.

The FAA disclosed in June that 300 NG and 737 MAX airplanes could contain improperly manufactured parts and said it would require these parts to be quickly replaced. The fine announced on Friday only relates to NG airplane components, the FAA said, but it is continuing to review the issue as it relates to the MAX.

The parts at issue are tracks on the leading edge of the wings used to guide the movement of slats that provide additional lift during takeoff and landing, the FAA said. The issue could result in a slat striking an airplane, potentially resulting in injury to passengers or preventing a safe landing.

The FAA said Boeing’s oversight failure “resulted in the installation of slat tracks that were weakened by a condition known as hydrogen embrittlement that occurred during cadmium-titanium plating.”

Boeing reiterated on Friday it had not been informed of any in-service issues related to the batch of slat tracks in question. It added that “all affected 737 NGs have been inspected and all slat track installations determined to be required have been completed on the NGs.”

The NG is the third-generation 737 that the company began building in 1997.

Boeing’s safety record has come under criticism from some in Congress, as has the FAA’s certification of the 737 MAX, which has been grounded since March after two fatal crashes killed 346 people. Boeing said it will ensure “all inspections and any necessary part replacements are performed on all 737 MAXs before they return to service.”