Radioactive groundwater at the crippled Fukushima nuclear plant has risen to levels above a barrier being built to contain it, according to Japanese media reports.

The Asahi newspaper said the revelations highlight the risk of an increasing amount of contaminated water reaching the sea, 

Citing data from a meeting yesterday of a Japanese regulator's task force working on the Fukushima clean-up, the newspaper reports that the contaminated water could swell to the ground surface within three weeks.

The latest revelation underscores the hurdles still facing Tokyo Electric Power (TEPCO) after the March 2011 earthquake and tsunami.

The disaster destroyed the Fukushima plant in March 2011, triggering the world's worst nuclear disaster since Chernobyl.

At yesterday's meeting a Tepco official said equipment to pump out the water should be ready in late August, the Asahi newspaper reports. 

No one at Tepco or the regulator, the Nuclear Regulation Authority (NRA), could be reached for comment.

The paper estimates Tepco would need to pump out about 100 tonnes 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% of its 380,000 tonnes of storage capacity is already filled, and Tepco has acknowledged it could run out of space.

Last month Tepco reversed months of denials and acknowledged that radioactive water has been leaking into the ocean.

One of its biggest challenges is trying to contain radioactive water that cools the reactors as it mixes with some 400 tonnes 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 metre below the surface, the Asahi newspaper said.