Japan's nuclear safety agency says two emergency workers at a crippled nuclear power plant have been exposed to more than twice the legally permitted level of radiation.

The Nuclear and Industrial Safety Agency said Friday a third worker from the Fukushima plant is undergoing further tests after initial findings showed high exposure.

The agency issued a warning against the plant's operator, Tokyo Electric Power Company, for violating the government-set limit on radiation exposure and ordered the company to draft measures to prevent similar incidents.

The Fukushima nuclear plant was damaged when a March 11 earthquake and tsunami knocked out the cooling systems at the plant.

Only in recent days has the Tokyo Electric Power Company admitted that nuclear fuel in three reactors at the plant appears to have melted down and burned through the bottom of their primary pressure chambers.

In another development Friday, Japan's government said survivors of the March 11 earthquake and tsunami need long-term mental health care. In its annual paper on suicide prevention, the government said survivors may be undergoing shock, stress and depression from the overwhelming losses. 

The report said they also feel may guilty for escaping death.

More than 23,000 people are dead or missing following the March natural disasters.