A fresh leak of radioactive water was detected at the damaged Fukushima nuclear plant on Sunday, raising new concerns about ongoing efforts to clean up the site.

Tokyo Electric Power Co. (TEPCO), which operates the Fukushima Daiichi plant, said sensors attached to a drainage channel that diverts rain and groundwater from the plant into the Pacific Ocean detected contamination levels up to 70 times greater than the already-high radiation levels previously documented at the facility.

According to Agence France-Presse, "the levels of beta ray-emitting substances, such as strontium-90, measured 5,050 to 7,230 becquerels per liter of water between 10:20 a.m. and 10:50 a.m. Tepco requires radioactivity levels of groundwater at the plant discharged into the sea to remain below 5 becquerels."

"The latest incident, one of several that have plagued the plant in recent months, reflects the difficulty in controlling and decommissioning the plant, which went through meltdowns and explosions after being battered by a giant tsunami in March 2011, sparking the world's worst nuclear disaster in a generation," AFP adds. "TEPCO has not been able to effectively deal with an increasing amount of contaminated water, used to cool the crippled reactors and molten fuels inside them and kept in large storage tanks on the plant's vast campus."

A TEPCO representative said its emergency inspections of those tanks storing nuclear wastewater did not find any additional abnormalities and that radiation levels had since been reduced to 10 to 20 times the normal levels. The company remains unclear on how the contamination occurred.

Just last week, the United Nations' International Atomic Energy Agency said Japan has made "significant progress" in cleaning up the site of the 2011 disaster.