The operator of the damaged Fukushima Daiichi nuclear power plant says more than 3 tons of radioactive water may have leaked from barriers surrounding storage tanks.
Tokyo Electric Power Company made the announcement on Friday following the discovery of water leaking around 2 of its storage tanks on the hillside earlier this week.
The tanks contain rainwater with radiation levels above government-set standards.
TEPCO found that regular patrols have not been conducted in the area near the tanks since March, and that the leakage may have begun then.
The company also determined that a drain valve for barriers surrounding the tanks had accidentally been left open.
It detected higher levels of radiation around the area than at other locations in the complex.
TEPCO has concluded that up to 3.4 tons of contaminated water may have leaked outside the barriers since March. The company continues to collect soil samples in the area.
TEPCO officials say regular patrols did not cover the area where only smaller tanks for rainwater are located. They add that they will thoroughly check for other facilities that may have been left unattended.