The operator of Japan's crippled Fukushima nuclear power plant said on Thursday that it is considering dumping water it treated for radiation contamination into the ocean as early as March, prompting protests from fishing groups.

Tokyo Electric Power, (Tepco) the utility operating Fukushima's Daiichi plant hit by a powerful tsunami in March in the world's worst nuclear accident in 25 years, said it was running out of space to store some of the water it treated at the plant due to an inflow of groundwater.

We would like to increase the number of tanks to accommodate the water but it will be difficult to do so indefinitely, Tepco spokesman Junichi Matsumoto told reporters, adding the plant was likely to reach its storage capacity around March.

The admission is a setback for the utility which appeared to be making progress in its cleanup after building a cooling system that no longer required pumping in vast amounts of water. It also built a system, drawing on French, U.S. and Japanese technology, that decontaminates the vast pool of tainted runoff to supply the cooling system with water.

The company said representatives of a nationwide federation of fishing cooperatives on Thursday visited its Tokyo headquarters to protest.

Tepco said it is still assessing the potential environmental impact of releasing the accumulating water, but that if forced to do so it would discharge water expected to have the least effect the environment.

Tens of thousands tons of water contaminated with radiation have accumulated at the plant, 240 kilometres (150 miles) northeast of Tokyo after early on in the crisis Tepco tried to cool reactors that suffered nuclear fuel meltdowns by pouring in water, much of it from the sea.

Our priority is also to look for ways to limit the inflow of groundwater into the buildings at the plant, Matsumoto said.

The operator estimates that due to the inflow the amount of water requiring storage is increasing by 200 to 500 tonnes every day.

The utility released more than 10,000 tonnes of water tainted with low levels of radiation in April to free up space for water that had much higher levels of radioactivity, drawing sharp criticism from neighbouring countries such as South Korea and China.

(Editing by Tomasz Janowski)