Japan's prime minister has promised "firm measures" to combat leaks of radioactive water from the Fukushima nuclear power plant after its operator, Tepco, was criticised for a lack of action.

"There is heightened concern among the public, particularly about the contaminated water problem," Shinzo Abe said during a government nuclear disaster response meeting at his office. "This is an urgent matter that needs to be addressed. The government will step in to take firm measures." Such measures might include funding a costly containment project.

The announcement came a day after Tepco admitted some of the water was seeping over or around an underground barrier it created by injecting chemicals into the soil. The Fukushima plant was crippled and went into multiple meltdown after 11 March 2011 earthquake and tsunami. Tepco has been battling to contain the situation every since.

The latest problem involves water that accumulated over the last month since Tepco began creating the chemical barrier. Government officials have said an estimated 300 tonnes of radioactive water has been leaking into the sea each day since early in the crisis.

Since a major leak from a maintenance pit a month after three reactors at the plant melted down, Tepco had denied any further leaks of radioactive water into the sea, despite repeated warnings by experts, until finally acknowledging them in July.

The underground barrier on the coastal embankment has slowed the leaks somewhat but has caused underground water to swell. To prevent an overflow above the surface, which is feared to happen within weeks, Tepco will start pumping out about 100 tonnes of underground water from coastal observation wells this week. Later this month it will remove old contaminated water from trenches near the coast, something it had left untouched despite repeated prodding from the government.

Government officials said Wednesday they were considering funding a separate, multibillion-dollar project to surround the reactor buildings with a wall of frozen ground to block underground water from entering the contaminated buildings. The project, announced in May, is scheduled for completion in July 2015.