A worker at Japan's disaster-stricken Fukushima nuclear power plant died Thursday, its operator said, adding that the death was not necessarily related to radioactive leaks.

The male worker, in his 50s, was taken to hospital for treatment Wednesday after feeling ill during a regular morning assembly at the plant, some 200 kilometres (140 miles) north of Tokyo, according to Tokyo Electric Power Co. (TEPCO).

He died early morning Thursday at the hospital, TEPCO spokeswoman Chie Hosoda said, adding that the cause of his death was being investigated.

"He had been exposed to a small amount of radiation. It is difficult to assume that radiation was a cause of his death," she said.

The unidentified worker had worked for 46 days at the plant to install a tank which will be used for processing contaminated water from the crippled reactor units.

He worked three hours every day and had been exposed to a total of 2.02 millisieverts of radiation, the official said. An exposure of 100 millisieverts per year is considered the lowest level at which any increase in cancer risk is evident.

He was the third worker to die at the plant following the March 11 earthquake and tsunami.

A male worker, in his 60s, died of a heart attack in May and another, in his 40s, succumbed to acute leukemia in August. TEPCO said both cases were not attributable to radiation.

Two other male employees were also killed directly in the disaster.

The 9.0-magnitude quake and tsunami left 20,000 dead or missing on Japan's northeast coast and crippled cooling systems at the Fukushima plant, causing reactor meltdowns.