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Japan -- Radiation exceeding government safety limits has seeped into groundwater under a tsunami-crippled Japanese nuclear plant, according to the operator, but experts said Friday that it was unlikely to contaminate drinking supplies.

The leak is, however, a concern and an indicator of how far the Tokyo Electric Power Co. is from stabilizing dangerously overheating reactors after cooling systems were knocked out in the March 11 tsunami.

TEPCO has increasingly asked for international help in its uphill battle, most recently ordering giant pumps from the U.S. that were to arrive later this month to spray water on the reactors.

The groundwater contamination was found in concentrations 10,000 times higher than the government standard for the plant. The iodine-131, a radioactive substance that decays quickly, was nearly 50 feet (15 meters) below one of the reactors, according to TEPCO spokesman Naoyuki Matsumo.

Seiki Kawagoe, an environmental science professor at Tohoku University, said the radioactive substances were unlikely to affect drinking water, noting that radiation tends to dissipate quickly in the ground, as it does in the ocean.

But there are two ways the iodine could eventually affect drinking water if concentrations were high enough. One is if it were to seep into wells in the area. For now, a 12-mile (20-kilometer) radius around the plant has been cleared, though residents of the area are growing increasingly frustrated with evacuation orders and have been sneaking back to check on their homes.

The other concern is that contaminated water from the plant could seep into underground waterways and eventually into rivers used for drinking water. Tomohiro Mogamiya, an official with the Ministry of Health, Labor and Welfare's water supply division, said that was "extremely unlikely" since groundwater would flow toward the ocean, and the plant is right on the coast.

There are two nearby filtration plants for drinking water, and both have been shut down because they are just inside the exclusion zone. One takes water from the Kido River, to the south, and another takes it from groundwater below Odaka, to the north. Both are several miles (kilometers) from the coast, and therefore on higher ground.