Efforts to pour water into reactors No.1, 2 and 3 at the crippled Fukushima nuclear complex to cool them down have been continuing in a stable manner after a major aftershock hit northern Japan on late Thursday, public broadcaster NHK said.

Japan's Nuclear and Industrial Safety Agency said separately that there are no irregularities in the nitrogen injection operation at Fukushima Daiichi's No.1 reactor, which was being conducted to prevent the risk of a hydrogen gas explosion.