The government has said that radiation levels have fallen at Japan's earthquake-stricken Fukushima Daiichi nuclear plant, reports the BBC.
The announcement was made after a fire was extinguished at the plant.
The government had earlier warned that radiation leaks from the plant had reached harmful levels.
Weather reports indicate winds are dispersing radiation from the plant to the east, over the Pacific Ocean, but the government has ordered people living within 20km (12 miles) to leave.
Officials have warned people within 20-30km of the plant to either leave the area or stay indoors.
Japan has also announced a 30km no-fly zone around the plant to prevent planes spreading the radiation further afield.
Further strong aftershocks - one of 6.1-magnitude centred south-west of Tokyo - continue to rock the country.
Friday afternoon's 9.0-magnitude earthquake, the strongest in Japan since records began to be kept, hit the north-east of the main island of Honshu and triggered a powerful tsunami that devastated dozens of coastal communities.
The latest official death toll from the quake and tsunami stands at nearly 3,400 - but thousands of people are missing and it is feared at least 10,000 may have been killed.
More than 500,000 people have been made homeless.
The government has deployed 100,000 troops to lead the aid effort.
The crisis at the Fukushima plant - which contains six nuclear reactors - has mounted since the earthquake knocked out the cooling systems.
Explosions rocked the buildings housing reactors one and three on Saturday and Monday.
On Tuesday morning a third blast hit reactor two's building. A fire also broke out at a spent fuel storage pond at the power plant's reactor four.
That reactor had been shut down before the quake for maintenance, but its spent nuclear fuel rods were still stored on the site.
Officials said the explosions were caused by a buildup of hydrogen.
Chief Cabinet Secretary Yukio Edano said they were closely watching the remaining two reactors at the plant, five and six, as they had begun overheating slightly.
He said cooling seawater was being pumped into reactors one and three - which were returning to normal - and into reactor two, which remained unstable.
Japan's nuclear safety agency said earlier it suspected the blast may have damaged reactor two's suppression chamber, which would have allowed radioactive steam to escape.
The BBC's Chris Hogg in Tokyo says that would make it a more serious incident than the previous explosions, which were thought just to have damaged the buildings housing the reactors.
Radiation levels in the Japanese capital - 250km away - were reported to be higher than normal, but officials said there were no health dangers.
Tokyo residents have been stocking up on supplies, with some stores selling out of items such as food, water, face masks and candles.
Housewife Mariko Kawase, 34, told AFP news agency: "I am shopping now because we may not be able to go out due to the radiation."
Radiation levels in Chiba prefecture, next to Tokyo, were 10 times above normal levels, Kyodo News reports.
After Tuesday's blast, radiation dosages of up to 400 millisieverts per hour were recorded at the Fukushima Daiichi site, about 250km north-east of Tokyo.
A single dose of 1,000 millisieverts causes temporary radiation sickness such as nausea and vomiting.
Three other nuclear power plants shut down automatically during Friday's earthquake and are in a safe and stable condition, the UN's nuclear agency reported on Tuesday.
The loss of so much generating capacity in one blow has meant rolling blackouts have had to imposed for parts of Tokyo.