Japan temporarily issued tsunami warnings for parts of the north-east coast on Monday following a powerful aftershock exactly a month after a magnitude-9 earthquake created huge waves that killed an estimated 28,000 people.
NHK, the public broadcaster, warned of tsunami up to 2 metres high on the coast of Ibaraki prefecture after the magnitude-7.1 quake.
Although the waves were estimated to be much smaller than those that devastated Japan's north-east coast on 11 March, the meteorological agency warned people in Ibaraki to evacuate to higher ground. The warnings were later lifted.
The aftershock came as the government announced it was widening the evacuation zone around the Fukushima Daiichi nuclear plant due to high levels of accumulated radiation and rising fears about the long-term effects on residents' health.
The government's chief spokesman, Yukio Edano, said the current 20km evacuation zone would be extended to five other communities, including the village of Iitate, which lies 40km from the plant.
The village's 5,000 residents were told to prepare to leave their homes, although the evacuation would not take place immediately as radiation levels presented no immediate threat to health.
The Asahi newspaper said the zone would be widened to 30km in some areas, depending on wind direction, adding that residents would be given about a week to prepare.
"We have made a new decision about evacuations based on data analysis of accumulated radiation exposure information," Edano told a news conference. "There is no need to evacuate immediately."
About 70,000 people living within 20km of Fukushima Daiichi have already been evacuated, and another 130,000 living inside a band 20-30km away have been told to leave voluntarily or remain indoors due to disruption to supply lines and services.
Engineers at the plant say that although progress has been made in containing radioactive water that has built up beneath reactors, they are still no closer to restoring damaged cooling systems.
Until repairs are made they cannot cool overheated fuel rods that could cause a major leak of radiation if they go into full meltdown.
The government had ignored pressure from the International Atomic Energy Agency and environmental groups to widen the evacuation zone. The US has advised its citizens not to go within 80km of the plant.
Hidehiko Nishiyama, a spokesman for Japan's nuclear safety agency, apologised for the anxiety caused by radiation leaks, and warned that the cooling systems could take months to repair.
"It's still difficult to give a timeline regarding when we can resolve the problem," Nishiyama said. "We are very sorry for the evacuees who are anxious to see the problem resolved."
The governor of Fukushima prefecture, Yuhei Sato, criticised the decision first to ask people in the area to stay indoors, then to consider leaving and now to prepare for evacuation. "Residents are very confused about what to do," he said.
Earlier, Sato refused to meet the president of the plant's operator, Tokyo Electrical Power, Masataka Shimizu, who had travelled to Fukushima to apologise to the prefecture's people.
The tsunami warning was a chilling reminder of the events of a month ago, which were marked across the country by a moment's silence at 2.46pm.
More than 14,000 people are still missing, and 152,000 survivors are living in evacuation centres.
Reflecting on the damage to his prefecture, which is also at the centre of the nuclear power crisis, Sato said: "My chest has been ripped open by the suffering and pain that this disaster has caused the people of our prefecture. I can't find the words to express my sorrow."
The prime minister, Naoto Kan, placed a message in newspapers in several countries, including Britain, China and the United States, thanking the international community for its support.
Kan said the generosity shown towards Japan in its time of need demonstrated the human capacity for kizuna, or bonds of friendship, and vowed that Japan would emerge a stronger nation.
"We deeply appreciate the kizuna our friends from around the world have shown and I want to thank every nation, entity, and you personally, from the bottom of my heart," he said.