TOKYO (AP) — The Japanese nuclear watchdog today said they are taking the leakage of highly radioactive water at the crippled Fukushima nuclear power plant seriously, and proposed raising the rating to describe it from “an anomaly” to a “serious incident.”

The operator of the plant said about 300 tons (80,000 gallons) of contaminated water has leaked from one of hundreds of steel tanks around the wrecked Fukushima Daiichi plant. Tokyo Electric Power Co. hasn’t figured out how or where the water leaked, but suspects it did so through a seam on the tank.

The leak, the fifth since last year involving tanks of the same design, also raised concerns that this could be the beginning of a new disaster — contaminated water leaking from storage tanks one after another.

“That’s what we fear the most. We must remain alert. We should assume that what has happened once could happen again, and prepare for more,” watchdog chairman Shunichi Tanaka told a news conference. “We are at a situation where there is no time to waste.”