The crippled Fukushima Dai-Ichi nuclear power plant in Japan has claimed its first death. But it wasn't due to radiation-related reasons. A worker involved in the plant's excavation work died after getting buried in a mudslide on Friday.

Tokyo Electric Power Co. said it was the first death attributed to a work accident since the March 2011 nuclear disaster.

Tatsuhiro Yamagishi, a spokesman for the utility, said the worker was stationed around a storage building, breaking up concrete when the ground collapsed. The mudslide trapped him under concrete and sediment.

The worker, a man in his 50s, was dug out and immediately rushed to hospital. Doctors failed to revive him to consciousness. He was pronounced dead three hours after the incident.

"Some danger is always involved," company spokesman Masayuki Ono told reporters. "We are deeply sorry."

"In the three years since the disaster, we had not had any worker deaths caused by work [inside the plant]. The fact that such a serious accident has occurred is deeply regrettable," Mr Ono added.

AP said Tepco is now conducting investigation to determine the specific cause of death.

The worker was hired by TEPCO through Toso Fudosan Kanri Company, a first-tier contractor, RT News said.

Friday's death comes after Tepco President Naomi Hirose was reminded by Japan's Nuclear Regulation Authority last week at a meeting to prioritise plant safety.

At least seven deaths have occurred in the crippled Fukushima Dai-Ichi nuclear power plant over the last three years. Three have earlier died from heart attacks and one from leukemia.

But the beleaguered Tepco continued to insist none of the deaths were related to radiation exposure. It pointed out workers were continuously monitored and immediately removed from duty once their bodies reach the annual radiation limit of 20 millisieverts.

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Three of Fukushima Dai-Ichi nuclear power plant's reactors went into meltdown and exploded in March 2011 when a tsunami set off by a magnitude 9 earthquake damaged the plant's cooling system.

Tepco had since employed thousands of workers to work on the plant's cleanup and decommissioning.