Latest Update on Japan Earthquake
More than 17,440 people are listed as missing, and 2,775 as injured. Hundreds of thousands remain homeless.
Meanwhile, the government says an investigation is under way to establish the source of the radiation leak at the quake-hit Fukushima nuclear plant, which left two workers in hospital.
Experts are checking whether the leak came from the reactor itself.
The water that caused the injuries to the two men on Thursday was 10,000 times more times radioactive than a normal sample; the men remain in hospital.
Japan’s Nuclear Safety Agency has reiterated its position that it believes the reactor may have been damaged but it is going no further than that, says the BBC’s Mark Worthington in Tokyo.
The agency has denied any suggestion that the reactor core may have cracked, Kyodo news agency reported.
The two workers had reportedly not been wearing the correct protective boots and had ignored a radiation alarm. A revision of safety measures at the Fukushima plant has been ordered.
In China, the authorities say two Japanese tourists found to have high levels of radiation arrived from Tokyo.
The Chinese government says they were sent to a specialist hospital following tests on their luggage and clothes that detected radiation levels “seriously exceeding limits”.
It remains unclear how the two may have become contaminated as neither traveller is reported to have been within 240km (150 miles) of the Fukushima plant, says the BBC’s Mark Worthington.
The Chinese news agency, Xinhua, has reported that abnormal radiation levels have been detected on a ship coming from Japan to Xiamen port in Fujian province.
Work is continuing at the stricken Fukushima Daiichi nuclear plant to restart generators to run vital cooling equipment at the six reactors.
The Tokyo Electric Power Company (Tepco), which runs the plant, has said a cold shutdown may still take a month.
“We are still in the process of assessing the damage at the plant, so that we can’t put a deadline on when the cooling operations will work again,” a Tepco spokesman told AFP.
The plant is 250km (155 miles) north-east of the capital, Tokyo. The government has declared a 20km exclusion zone and evacuated tens of thousands of people.
Those living up to 30km away have been told to stay indoors to minimise exposure.
Radiation levels in Tokyo’s water supply have now fallen, but remain high in other areas of northern Japan.
Japan has banned shipments of foodstuffs from areas around the damaged nuclear plant.
People in Fukushima prefecture have been told not to eat 11 types of green leafy vegetables grown locally because of contamination worries.
Demand at Tokyo’s usually busy Tsukiji fish market has dropped dramatically, reports say.
Importers of Japanese products are finding low levels of radiation in some food stuffs; though unsettling, the amounts found have in no cases been life-threatening.
Singapore’s Agri-Food and Veterinary Authority said it had found small levels of contamination in Japanese mustard, parsley and two other plants imported from the prefectures of Tochigi, Ibaraki, Chiba and Ehime.
Singapore, Hong Kong, and other Asian importers have already placed bans on the import of vegetables, seafood and milk products from Fukushima, Ibaraki, Tochigi and Gunma prefectures.
Australia, the European Union, the United States and Russia have followed suit.
Shipping has also been affected: Hapag-Lloyd container shipping line is among a handful of companies that have said they will temporarily avoid sailing to Yokohama Port, inTokyo Bay.
Most of Japan’s oil terminals remain open, Inchcape Shipping Services was quoted as saying.
Although damaged by the tsunami, 12 out of 15 ports in north-eastern Japan have started functioning again.
At least 18,000 houses were destroyed and 130,000 damaged by the quake and ensuing tsunami, and about 250,000 people are living in emergency shelters.
The Japanese government has said it will cost as much as 25 trillion yen ($309bn; £189bn) to rebuild the country after the disaster.