Tepco, the utility that owns the plant hit by a tsunami in 2011 that set off meltdowns, has been widely criticised for its treatment of workers and its handling of the cleanup, which is expected to take decades.

In a statement on Thursday, the U.N. experts urged Japan to act urgently to protect tens of thousands of the workers from reported exploitation and radiation exposure, citing fears over possible coercion and adequate training and protective steps.

Japan is conducting reliable management of radiation levels for Fukushima workers, however, and had already informed the office of the United Nations High Commissioner for Human Rights of this, along with data, the Foreign Ministry said on Friday.

"The fact that this statement was issued despite this is extremely regrettable, and this was conveyed to the OHCHR in Geneva on Thursday," it added.

The U.N. rights experts had said they were engaged in a dialogue with Japan since last year, with the government accepting to "follow up" on some recommendations.

A Reuters investigation in 2013 had found widespread labor abuses, including workers who said their pay was skimmed and spoke of scant scrutiny of working conditions. Tepco, or Tokyo Electric Power Co Holdings, said at the time it was taking steps to limit worker abuse.

REUTERS