People with suspected radiation contamination are being evacuated
Technicians are battling to stabilise a third reactor at a quake-stricken Japanese nuclear plant, which has been rocked by a second blast in three days.
The Fukushima Daiichi plant's operators have resumed pumping seawater into reactor 2 after a cooling system broke.
They warned of a possible meltdown when the fuel rods became exposed after the pump stopped as its fuel ran out.
A cooling system breakdown preceded explosions at the plant's reactor 3 on Monday and reactor 1 on Saturday.
The latest hydrogen blast injured 11 people, one of them seriously, and sent a huge column of smoke billowing into the air.
Mass evacuation
The Tokyo Electric Power Co (Tepco) is playing down any health risk, saying thick containment walls shielding the reactor cores have remained intact.
Experts say a disaster on the scale of Chernobyl in the 1980s is highly unlikely because the reactors are built to a higher standard and have much more rigorous safety measures.
But the US said it had moved one of its aircraft carriers from the area after detecting low-level radiation 160km (100 miles) offshore.
Tens of thousands of people have been evacuated from a 20-km exclusion zone around the Fukushima Daiichi plant.
In other developments:
- Two thousand bodies have been found on the shores of Miyagi prefecture, Japanese media are reporting
- The central bank said it would pump 15 trillion yen ($182bn; £113bn) into the economy to prop up markets, but the Nikkei slumped more than 6%
- Prime Minister Naoto Kan postponed planned rolling power cuts, saying they may not be needed if householders could conserve energy
The relief operation is continuing after Friday's magnitude 8.9 quake, which triggered a tsunami that devastated swathes of the north-eastern coast of the country.
The death toll remains unclear - officials in Miyagi estimate 10,000 people died in that prefecture alone.
Tens of thousands of relief workers, soldiers and police have been deployed to the disaster zone.The government asked people not to go to work or school on Monday because the transport network would not be able to cope with demand.
The capital, Tokyo, is still experiencing regular aftershocks, amid warnings that another powerful earthquake is likely to strike very soon.
The disaster is a huge blow for the Japanese economy - the world's third largest - which has been ailing for two decades.
The Foreign Office has updated its travel advice to warn against all non-essential travel to Tokyo and north-eastern Japan.
