An operation to reduce the risk of a hydrogen explosion at the crisis-hit Fukushima nuclear power plant by injecting nitrogen and pouring coolant water into reactors at the site was unaffected by a strong quake that hit northeastern Japan on Thursday night, the plant operator and the government's nuclear agency said.

A quake with a preliminary magnitude of 7.4 caused engineers engaged in recovery work at the Fukushima Daiichi station to temporarily evacuate, but no one was injured and no abnormalities have so far been detected in radiation levels near the plant, they said.

The workers are expected to inspect the site, which was ravaged by a magnitude 9.0 quake and tsunami on March 11, now that a tsunami warning issued for the area has been lifted.

Highly radioactive water that has been filling up underground trenches at the plant did not overflow, the government's Nuclear and Industrial Safety Agency said.