Ken Saito, Japan's minister of economy, trade and industry, told reporters on Tuesday that he would instruct TEPCO President Tomoaki Kobayakawa to ensure the safety of the decommissioning process. "Even a single mistake could result in a loss of trust from the local community and society," Saito said.

The warning comes half a year after the government and TEPCO started to release treated water from the plant into the Pacific Ocean. The activity has sparked safety concerns in other countries.

The operator said on Feb. 7 that 5.5 tonnes of water, estimated to contain 22 billion becquerels of radioactive substances, had leaked from a building for treating contaminated water earlier that day. This raises questions over TEPCO's ability to safely decommission the quake-wrecked plant.

TEPCO said the recent incident was caused by a worker's error in closing a piping valve, but added that no impact had been detected on staff health or the environment outside the plant.

The company is suspected of violating work rules and safety measures legally prescribed and outlined in its decommissioning implementation plan. Japan's Nuclear Regulation Authority said on Monday that TEPCO failed to manage the work properly as it had not made clear which department was responsible for handling the valve. The watchdog said it would continue to look into the matter.

TEPCO was previously called out for violating the plan last year, after workers at the Fukushima plant were exposed to radioactive liquid waste in October. The company said it would work to prevent such cases occurring again.