Japan fought on Sunday to avert a disastrous meltdown at two earthquake-crippled nuclear reactors as estimates of the death toll from the tsunami that charged across its northeast rose to more than 10,000.
Broadcaster NHK, quoting a police official, said more than 10,000 people may have been killed as the wall of water triggered by Friday's 8.9-magnitude quake surged across the coastline, reducing whole towns to rubble.
Officials worked desperately to prevent the fuel rods in the damaged plants from overheating after radiation leaked into the air. The government said a building housing a second reactor was at risk of exploding after a blast blew the roof off a different plant the day before.
The nuclear accident, the worst since the Chernobyl disaster in 1986, sparked stinging criticism that authorities were ill-prepared for such a massive quake and the threat that could pose to the country's nuclear power industry.
Thousands were evacuated on Saturday following the explosion and leak from the facility's No. 1 reactor in Fukushima, 240 km (150 miles) north of Tokyo, where there is believed to have been a partial meltdown of the fuel rods.
Engineers were pumping in seawater, trying to prevent the same happening at the No. 3 reactor, the government said in apparent acknowledgement it had moved too slowly on Saturday.
"Unlike the No.1 reactor, we ventilated and injected water at an early stage," Chief Cabinet Secretary Yukio Edano told a news briefing.
Asked if fuel rods were partially melting in the No. 1 reactor, Edano said: "There is that possibility. We cannot confirm this because it is in the reactor. But we are dealing with it under that assumption ."
He said fuel rods may have partially deformed at the No. 3 reactor but a meltdown was unlikely to have occurred.
"The use of seawater means they have run out of options," said David Lochbaum, director of the Union of Concerned Scientists Nuclear Safety Project.
"If they had any other water they would have used it. It likely means the power for their pumps is gone."
Nuclear plant operator Tokyo Electric Power (TEPCO) said radiation levels around the Fukushima Daiichi plant had risen above the safety limit but that it did not mean an "immediate threat" to human health.
Edano said there was a risk of an explosion at the building housing the No. 3 reactor, but that it was unlikely to affect the reactor core container.
The government said it planned electricity blackouts in areas covered by TEPCO lasting a few weeks.
The disaster prompted an angry response from an anti-nuclear energy NGO in Japan which said it should have been foreseen.