Work at Unit Two is considered the most pressing as radioactive water, which was used to urgently cool the reactor and fuel storage facilities when the plant's cooling systems failed after the March earthquake, has been leaking into the turbine building and adjacent facilities.

The company said the level of contaminated water in the Unit Two reactor is one centimeter lower than on Tuesday.

TEPCO has started pumping radioactive water into storage tanks, the Kyodo news agency said on Tuesday, citing company sources.

The Nuclear and Industrial Safety Agency has reported to a Cabinet safety panel that nuclear fuel pellets in the No 1 to 3 reactors at the quake-hit Fukushima power station are believed to have partially melted, Japanese Daily Yomiuri Online said.

NHK cited TEPCO as saying that contaminated water levels are also rising in the basements of reactors No 5 and 6, and in tunnels connected to reactors No.3 and 4.

Hidehiko Nishiyama, a spokesman for the agency, said NISA is considering the construction of fortifications under a pool for nuclear waste at the fourth Unit due to continuing earthquakes in the country.

The water, containing high concentrations of radioactive cesium and iodine, is now being pumped out with an 800-meter hose into a nuclear storage facility. Workers will have to pump up to 10,000 cubic meters of highly contaminated water from the reactor building by April 26.

Japan's Fukushima Daiichi Nuclear Power Plant was seriously damaged by the powerful earthquake and tsunami that hit Japan on March 11. Fukushima's operator has since been struggling to stop radioactive leaks from the plant's crippled reactors.

According to most recent reports, the death toll from the 9.0-magnitude earthquake and tsunami stood at 13,949 as of Tuesday morning, with 13,678 still listed as missing.