Nippon Steel, the world's No. 4 steelmaker, is looking to buy more stakes in coking coal mines to secure stable supply of the key steel-making ingredient, its executive said.

Japan's biggest steelmaker already owns stakes in several coking coal mines and iron ore mines, procuring about 20% of the 27 million tons of its annual import of coking coal and the 58 million tons in iron ore import from those holdings.

"It's not necessary to stop at the 20%," said executive vice president Takahiro Mori on Tuesday in an interview.