Radiation detected in the quake-hit Japanese capital's tap water exceeds safe level for infants, it is being reported.
The radioactive iodine levels in Tokyo's water was announced by a government official, following tests by city authorities.
"Under government guidelines, water containing a radioactive substance of more than 100 becquerels per kilogramme should not be used for milk for babies," he said.
In one Tokyo ward, a water sample contained more than double the legal limit, at 210 becquerels per kilogramme, he added.
It came as workers at the country's leaking nuclear complex hoped to bring an end to the plant's crisis after hooking up power lines to all six of its reactor units.
We must overcome this crisis that we have never experienced in the past, and it's time to make a nationwide effort.
Japan's chief cabinet secretary Yukio Edano
They will now try to restart the overheated Fukushima site's crucial cooling system, which was knocked out by a tsunami that devastated the country's northeast coast.
The earthquake and tsunami, Japan's worst crisis since World War II, have left nearly 23,000 people dead or missing, with entire coastal communities swept away.
Officials say radiation is still leaking from the nuclear plant, which lies just 155 miles (250km) from Tokyo and its 30m inhabitants.
Some food imports from quake and tsunami-hit Japan have now been banned in the US due to fears of radiation and nuclear contamination.
American authorities have placed an import alert on all milk products, fresh vegetables and fruits from Fukushima, Ibaraki, Tochigi and Gunma regions.
US secretary of state Hillary Clinton said: "What is most important is making sure that we help Japan deal with the after-effects of whatever occurred inside the reactors and that we also make sure the Japanese people have all the food that they need during this transition period."
She said the ruling was "as much focused on determining what is or isn't safe for the Japanese people, not just what is safe for export."
The World Health Organisation previously issued a warning over certain Japanese food products.
The crisis also continues to batter Japan's once-robust economy, with three of the of the country's biggest brands - Toyota, Honda and Sony - badly hit.
Each has been forced to delay a return to normal production due to shortages of parts and raw materials because of earthquake damage to factories in affected areas.
The National Police Agency in Japan said the overall number of bodies collected so far stood at 9,099.
An additional 13,786 people have been listed as missing, though there may be some overlap on those two lists.
Chief cabinet secretary Yukio Edano said: "We must overcome this crisis that we have never experienced in the past, and it's time to make a nationwide effort."