Officials with Japan's transport ministry have inspected plants run by a subsidiary of engineering giant IHI as part of an investigation into falsified fuel efficiency data for engines.

Officials visited two plants of IHI Power Systems, one in Niigata Prefecture and the other in Gunma Prefecture, on Thursday morning.

IHI admitted Wednesday that its subsidiary had falsified fuel efficiency data for more than 4,300 engines shipped from 2003 onwards.

IHI says the affected units are engines for fishing boats and tugboats, as well as engines used in power generation systems and trains.

Ministry officials are expected to interview managers at the company to learn how the falsification started.