Japan has been rattled by a 6.1-magnitude aftershock, with an epicentre 150 kilometres northeast of Tokyo and just south of the troubled nuclear plants leaking radiation.

There's no word yet on how the Saturday night tremor, which shook buildings in the capital, is affecting attempts to deal with the overheated Fukushima Daiichi nuclear complex.

The 9.0 quake and subsequent tsunami that ravaged Japan's northeastern coast March 11 knocked out backup cooling systems at the nuclear plant, which has been leaking radiation.

In another development, firefighters finished their 13-hour operation to cool the No.3 reactor at the plant early Sunday morning.

The Tokyo Fire Department started spraying water Saturday afternoon, setting up an unmanned vehicle in front of the reactor building. Its 800-metre hose sprayed sea water into a pool containing spent fuel rods.

Emergency crews worked Saturday to restore electrical power to the plant so they can try to restart the systems that cool the nuclear reactors.

Tokyo Electric Power Co., the company trying to get the reactors under control, said it has managed to lay power lines to four units at Fukushima Daiichi.

By Sunday, the company said, it may have a line to the most dangerously hot unit, but there are no guarantees its cooling pumps will work after a week of explosions, the CBC's Curt Petrovich reported.

Food contamination

In another development, spinach and milk from farms near the stricken nuclear plant show radiation above the safety limit, and some tap water is showing traces of radioactive iodine, an official said Saturday.

Yukio Edano, the chief cabinet secretary, said more foods will be checked, but what has been discovered so far poses no immediate health risk. He said a person would have to drink the milk for a year to ingest as much radiation as in a CT scan. A year of the spinach would amount to about one-fifth of a CT scan.

"It's not like if you ate it right away you would be harmed," Edano told reporters in Tokyo as Japan's nuclear crisis entered its second week. "It would not be good to continue to eat it for some time."

Concern over tap water

Also worrisome were the traces of radioactive iodine detected in tap water in Tokyo, 220 kilometres from the overheating nuclear plant in Fukushima prefecture, and in several other prefectures, although again the government played down any safety concern.

The iodine in Fukushima tap water tested above safety limits on Thursday, the government said, but had fallen to within legal limits by Saturday.

The food and water reports were the first since the March 11 quake and tsunami knocked out power to cooling systems at the Fukushima Daiichi nuclear complex. Four of the plant's six reactor units have experienced fires, explosions or partial meltdowns.

The contaminated milk was found on farms 30 kilometres from the plant, and the spinach was collected between 80 and 100 kilometres to the south, Edano said.

The government has banned sales and exports of food from the areas where the tainted food was found.

Leaking radiation

UN radiation tracking shows levels taken elsewhere in Japan, as well as in Russia and California, are minuscule, a diplomat with access to the readings said.

The government has admitted it was slow to respond to the nuclear troubles following last week's quake and tsunami, which has killed at least 7,000 people and displaced more than 400,000 others.

Edano said Saturday that the Fukushima Daiichi nuclear complex is deeply troubled, but things aren't getting worse.

Eriko Ohara, who lost her husband in Japan's earthquake and tsunami eight days ago, feeds her child at an evacuation centre in the devastated town of Kaname Muto.

"The situation at the nuclear complex still remains unpredictable. But at least we are preventing things from deteriorating."

Officials said late Saturday the water temperature is dropping in the spent fuel rod pool of the No. 5 reactor at the plant.

Tokyo Electric Power Company restored a power generator at the No. 6 reactor on Saturday morning. Now there are two generators working with enough power to maintain the cooling functions of the No. 5 and No. 6 reactors.

Kenji Kawasaki, a spokesman for Japan's nuclear safety agency, said a fire truck with a high-pressure cannon was outside the plant's Unit 3, shooting water non-stop into a cooling pool.

The CBC's David Common reported that the firefighters involved in the operation could only spend short amounts of time at the truck, "because the radioactive levels are just so very high, as you are that close to the nuclear cores which are damaged."

Emergency workers also funnelled water into the most troubled reactors — Units 1, 2 and 3, officials said.

A power company official said holes were punched in the roofs of the buildings housing Units 5 and 6, as workers tried to prevent dangerous buildups of hydrogen gas — a sign that temperatures continued to rise in those units' fuel storage pools.

Firefighters had started pumping water into Unit 5's pool, and the temperature had gone down, but a pump broke, delaying the refilling, the official said.

Backup failed

Meanwhile, Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency said backup power systems at the plant had been improperly protected, leaving them vulnerable to the tsunami.

The failure of Fukushima's backup power systems, which were supposed to keep cooling systems going after a quake, let uranium fuel overheat and were a "main cause" of the crisis, Nishiyama said.

"I cannot say whether it was a human error, but we should examine the case closely," he told reporters.

A spokesman for Tokyo Electric Power said that while the generators themselves were not directly exposed to the waves, some electrical support equipment was outside.

With files from The Associated Press