The containment vessel of the quake-hit No. 3 reactor of the Fukushima No. 1 nuclear power plant may have been damaged, Japan's top government spokesman said Wednesday, indicating that radioactive steam may be spreading outside.

The government's nuclear safety agency said the radiation level briefly reached 10 millisievert per hour at the plant's entrance at 10:40 a.m., but added that it was possibly due to radioactive substances emitted from the No. 2 reactor, as the government and Tokyo Electric Power Co. continued to scramble to address the ongoing nuclear crisis in the wake of Friday's devastating earthquake that hit Japan centering on its northeastern region.

The utility firm is also considering spraying boric acid by helicopter to prevent spent nuclear fuel rods at another troubled reactor, No. 4, from reaching criticality again, restarting a chain reaction.