Major Japanese heavy machinery-maker IHI Corp. has reported 6,340 cases of improper maintenance checks of engines for commercial aircraft, a dramatic jump from the 211 announced in its interim investigation report on March 8.

The surge came as the company added engines of aircraft used by foreign airlines in its probe into the irregularities, it said Friday. The interim report covered only engines of airplanes operated by domestic carriers.

The industry ministry imposed an administrative penalty on the company based on the aircraft manufacturing business act. IHI was ordered to conduct engine maintenance and repair work under approved methods. It is very rare for an administrative penalty to be doled out based on the law.

The number of cases of improper aircraft engine checks at IHI may continue to grow as the transport ministry continues its investigation.

IHI said it has confirmed that the irregularities do not affect the safety of aircraft concerned.

Between January 2017 and this January, the company carried out maintenance checks on a total of 212 engines used in planes operated by domestic and foreign carriers. Of them, 209 engines were checked by unqualified workers, and inspection documents were falsified using seals of qualified workers, according to the firm. Such fraudulent use of qualified inspection workers’ seals were found in 5,846 of the overall cases. The remaining cases involved such misconduct as unauthorized changes in inspection procedures and manipulation of inspection dates.

The irregularities at IHI were uncovered during an on-site inspection by the transport ministry.