TOKYO - High levels of toxic strontium-90 have been found in groundwater at the devastated Fukushima nuclear power plant in Japan, the utility that operates the facility said on Wednesday.

Strontium-90 is a by-product of the fission of uranium and plutonium in nuclear reactors as well as nuclear weapons, according to the website of the US Environmental Protection Agency.

It was not immediately clear how much of a setback the discovery would be for efforts by Tokyo Electric Power to clean up the plant, which was devastated when an earthquake and tsunami two years ago caused three reactor meltdowns.

Officials from the utility, known as Tepco, told a news conference that testing of groundwater outside the turbine building of reactor No 2 had shown that the level of strontium-90 had increased by more than 100 times between Dec 2012 and May this year.

Tepco said it was likely the radioactive material entered the environment after water poured over the melted fuel in unit No 2 and leaked out via the turbine building, which is located between the reactor and the ocean.

The elevated readings of strontium are more than 30 times the legal limit.

Tepco has struggled with the clean-up of Fukushima, the site of the world's worst nuclear disaster in 25 years, since a magnitude 9.0 earthquake and tsunami knocked out power and cooling to the station.

Recent mishaps, including two power outages, have heightened concerns about Fukushima's stability and called into question Tepco's ability to decommission the plant.