TOKYO — Foreign nuclear experts harshly criticized the operator of the devastated nuclear power plant at Fukushima on Friday for its delay in disclosing that highly contaminated groundwater has been leaking from the site into the ocean.

The operator, Tokyo Electric Power Company, or Tepco, announced the leaks on Monday, but only after denying for weeks that radioactive water was flowing into the Pacific. The lack of transparency has renewed frustration among a public still angry at Tepco for repeatedly underplaying the dangers posed by the plant after the triple meltdown there in 2011.

“This poor communication program gives the impression of a lack of an effective decision-making process, a lack of ability of keeping the people of Japan informed, and it brings into question whether Tepco has a plan and is doing all it can to protect the environment and the people,” said Dale Klein, a nuclear expert who heads a committee hired by the utility to recommend changes in its corporate culture.

The rebukes came as the company acknowledged that a reactor at the plant was again releasing radioactive steam, and said it still did not know the cause. The company first announced last week that it had spotted steam, but then said the release had stopped. Experts have said it is unlikely that the nuclear fuel is experiencing renewed fission, considered the worst-case scenario. But the release is still thought to be worrisome because it is venting radioactive materials into the air. Tepco has said the steam is not highly contaminated.