A powerful earthquake has hit north-east Japan, exactly one month after the devastating earthquake and tsunami.
The 7.1-magnitude tremor triggered a brief tsunami warning, and forced workers to evacuate the crippled Fukushima nuclear plant.
The epicentre of the quake was in Fukushima prefecture, and struck at a depth of just 10km (six miles).
It came as Japan said it was extending the evacuation zone around the nuclear plant because of radiation concerns.
The cooling systems at the Fukushima Daiichi nuclear plant were damaged in last month's disaster. Workers have been struggling to prevent several reactors from overheating, and avert a large-scale release of radiation.
The plant's operator, Tepco, said power used to pump water to cool three damaged reactors had been cut briefly but early indications suggested the plant had not sustained any further damage.
The zone around it will be widened to encompass five communities beyond the existing 20-km (12-mile) radius, following new data about accumulated radiation levels, officials said.
Top government spokesman Yukio Edano said the new evacuations would take place over the coming month, from areas including Iitate village, which lies 40km from the power station, and part of the city of Kawamata.
"This is not an emergency measure that people have to evacuate immediately," he told a news conference, but added that there were concerns about long-term health risks.
The latest tremor struck shortly after the country stopped to observe a minute's silence to remember the nearly 28,000 dead or missing in the 11 March disaster.
Survivors in shelters marked the moment the quake and tsunami hit at 1446 (0546 GMT) with bowed heads.
Prime Minister Naoto Kan thanked people around the world for their support.
In an open letter carried in seven newspapers around the world, he said that the support had brought hope and inspired courage at a desperate time.
"Through our own efforts and with the help of the global community, Japan will recover and come back even stronger. We will then repay you for your generous aid," he wrote.
"With this in our hearts, we now stand together dedicated to rebuilding the nation."
The official death toll from the disaster is 13,130, while 13,718 remain unaccounted for.
More than 150,000 people have been made homeless. Many have lost their jobs.
The prime minister has tried to reassure survivors that the fishing industry - which many in the area rely on for their livelihoods - would resume as soon as possible.
The tsunami wrecked boats and piers, closing down big fishing operations.
But the damage to the nuclear plant has also hit the fishing industry, as public and international buyers ditch Japanese food products over fears of contamination.
During a visit to Fukushima on Monday, Tepco chief Masataka Shimizu apologised for the nuclear accident.
The people who live near the plant are "suffering physically and mentally due to the nuclear radiation leak accident," he said.
"We sincerely apologise for this," he said.
The local community has been so angered by Tepco's handling of the incident that Fukushima's mayor refused to meet Mr Shimizu.
Workers have been feeding water into three reactors at the plant to cool fuel rods.
They are continuing to inject nitrogen into the No 1 reactor to prevent another blast caused by a build-up of hydrogen gas.
They have also been releasing water with low levels of radioactivity into the sea so that they can transfer highly radioactive water to a sealed area on site.
Officials have warned it will be several months before the situation at the nuclear facility is brought fully under control.