Radiation from Japan's crippled nuclear power plant has seeped into groundwater, the operator of the plant, TEPCO, reported Friday.

Whether the spreading radiation will contaminate sources of drinking water for residents living in the vicinity of the Fukushima nuclear plant, remains unknown, though Japanese experts believe it was unlikely.

The groundwater pollution by the radiation proves to be another serious setback for TEPCO, which has tried all measures to cool the overheating four reactors there after the March 11 monster quake and tsunami.

The groundwater radioactive level was found in concentrations 10,000 times higher than the government-set standard for the nuclear industry. The iodine-131, a radioactive substance that decays quickly, was nearly 15 meters below one of the reactors, said TEPCO spokesman Naoyuki Matsumo.

Two nearby filtration plants for drinking water have been shut down, as they are located just outside the 20-km exclusion zone, officials said.

In the latest report of food becoming tainted, the government said Friday that a cow slaughtered for beef had slightly elevated levels of cesium, another radioactive particle. Officials stressed that the meat was never put on the market.

Radioactive cesium can build up in the body and high levels are thought to be a risk for various cancers.

Contamination has also affected work at the nuclear plant itself, where radioactive water has been pooling, often thwarting the vital work of powering up the complex's cooling systems.

Though the company has acknowledged that it was initially slow to ask for help in dealing with the nuclear crisis, experts from around the world are now flooding in. French nuclear giant Areva, which supplied fuel to the plant, is helping figure out how to dispose of contaminated water, and American nuclear experts are joining Japanese on a panel to address the disaster.