On Wednesday, a government official said up to 300 tonnes of highly radioactive water may be flowing into the sea every day.

Prime Minister Shinzo Abe said the government would take "firm measures" to contain the leak.

One proposal being considered is to freeze the ground around the plant.

The plant was badly damaged by an earthquake and tsunami in 2011. It has been hit by a spate of water leaks and power failures in recent months.

"Rather than relying on [plant operator] Tokyo Electric, the government will take measures," Mr Abe told reporters on Wednesday.

"This is an urgent matter that needs to be addressed."

Penn Bowers, a research analyst at CLSA Asia Pacific in Tokyo, told the BBC that the government appeared to be set to take a bigger role in the Tepco clean-up.

"[The government has] invested capital in the company to date but not taken any direct operational steps at the facility until now," he said.

Contaminated water may have been leaking from the plant into the sea ever since the 2011 nuclear disaster, an official at Japan's energy ministry said.

The government believed that up to 300 tonnes of radioactive water could be leaking into the sea on a daily basis, the official added.

However, plant operator Tokyo Electric Power Company (Tepco) told the BBC that the figure was only a guess, and that it was not clear how much water flowing into the sea was contaminated with radiation.

The contaminated water is thought to have come from the 400 tonnes of groundwater pumped into the plant every day to cool the reactors, after cooling systems were knocked out by the tsunami.

The government is discussing proposals to build a wall of frozen ground around the reactor buildings to prevent further leaks. Reports suggest this could cost up to 40bn yen ($400m; £260m).