Nuclear safety spokesman Hidehiko Nishiyama says the air above the leak contains 1,000 millisiverts of radioactivity. Exposure to 500 millisiverts over a short period of time can increase the risk of cancer.
The water was seeping Saturday from a crack in the containment for a maintenance pit on the edge of the nuclear site.
The Fukushima Dai-ichi plant has been spewing radioactivity since it was hit by a tsunami three weeks ago.
Earlier Friday, Japan's nuclear safety agency ordered a review of the latest radiation measurements around the Fukushima nuclear plant.
The agency said the latest unsafe levels seem suspiciously high, and there may be a problem with the computer program used to analyze the readings.
One of the questionable measurements shows groundwater at the plant has 10,000 times the amount of radiation considered safe.
The company that runs the Fukushima plant has already had to retract some unsafe readings.
Earlier this week, one measurement mistakenly showed water inside the reactor was 10 million times higher than normal.
Japanese leaders, though, said they're not taking any chances and will take a closer look at how much radiation may be leaking into the water around that plant.
"Underground water seems to contain some level of radioactive substances and this leads to an understanding that the sea water or the soil in the vicinity needs to be monitored closely as well," said Japanese Chief Cabinet Secretary Yukio Edano.
Meantime, a U.S. company is preparing to send special equipment to help control the Fukushima nuclear plant.
Two giant concrete pumps, described as the largest in the world, will be used to pour water on the damaged reactor.
They are being retrofitted this week to spray water instead of concrete.
This week, traces of the radiation from Japan showed up in milk produced at U.S. dairies. Those levels are still well below anything considered dangerous, but some suppliers are taking extra steps to prevent any problems.
Fish wholesalers in downtown Los Angeles are running extra tests on everything imported from Japan.
"We've actually sent, voluntarily sent fish, farm-raised fish to Arizona nuclear testing companies. They've all come out negative," said Ray Watanabe, a fish wholesaler.
In addition to screenings in the U.S., many imports also are tested before they leave Japan.
Of course the problems at the nuclear plant are just one part of the massive recovery effort facing Japan. Starting Friday, U.S. and Japanese military forces have teamed up for a special search operation.
They're conducting searches to find the bodies of tsunami victims.
Although more than 11,000 people have been confirmed dead, another 16,000 are still missing.
The Associated Press contributed to this report.