Radioactive groundwater at the crippled Fukushima nuclear plant has risen to levels above a barrier being built to contain it, thus rising toward the surface and exceeding legal limits of radioactive discharge, Shinji Kinjo, head of a Nuclear Regulatory Authority (NRA) task force.

The Asahi newspaper, citing data from a Friday meeting of a task force working on the Fukushima clean-up at Japan's nuclear regulator, estimated that the contaminated water could swell to the ground surface within three weeks.

The latest revelation underscores the hurdles facing Tokyo Electric Power (TEPCO) two and a half years after a massive earthquake and tsunami destroyed the Fukushima plant, triggering the world's worst nuclear disaster since Chernobyl.

One of TEPCO's biggest challenges is trying to contain radioactive water that cools the reactors as it mixes with some 400 metric tons of fresh groundwater pouring into the plant daily.

TEPCO has been injecting a chemical into the ground to build barriers to contain the groundwater. But the method is only effective in solidifying the ground from 1.8 meters below the surface, whereas data from test wells shows the contaminated water has risen to one meter below the surface, the Asahi said.

The Asahi noted that TEPCO would need to pump out about 100 metric tons of water each day to prevent leakage into the ocean but that it was not clear where the water would be stored. More than 85 percent of its 380,000 metric tons of storage capacity is already filled, and TEPCO has acknowledged it could run out of space.