TOKYO — At least 45 tons of highly radioactive water have leaked from a purification facility at the Fukushima Daiichi nuclear power station, and some of it may have reached the Pacific Ocean, the plant’s operator said Sunday.

Nearly nine months after Fukushima Daiichi was ravaged by an earthquake and tsunami, the plant continues to pose a major environmental threat. Before the latest leak, the Fukushima accident had been responsible for the largest single release of radioactivity into the ocean, threatening wildlife and fisheries in the region, experts have said.

The new radioactive water leak called into question the progress that the plant’s operator, Tokyo Electric Power Company, appeared to have made in bringing its reactors under control. The company, known as Tepco, has said that it hopes to bring the plant to a stable state known as a cold shutdown by the end of the year.

The trouble on Sunday came in two stages, a Tepco statement said. In the morning, utility workers found that radioactive water was pooling in a catchment next to a purification device; the system was switched off, and the leak appeared to stop. But the company said it later discovered that leaked water was escaping, possibly through cracks in the catchment’s concrete wall, and was reaching an external gutter.