A Japanese lawmaker has drunk a glass of water taken from a radioactive puddle inside a reactor building at the stricken Fukushima nuclear plant in a bid to prove decontamination efforts are working.

Television footage showed a visibly nervous Yasuhiro Sonoda gulping down water that he said had been decontaminated after being scooped up from inside the plant.

"Just drinking (decontaminated water) doesn't mean safety has been confirmed, I know that," Sonoda told reporters. "Presenting data to the public is the best way."

Sonoda, parliamentary secretary for the cabinet office, said Monday that he drank the water after journalists repeatedly asked him to "prove" the area around the plant was safe.

The water came from puddles that had collected in the plant following clean-up efforts, a source not normally intended for human consumption.

Radioactive contaminants spewed into the environment from the Fukushima facility following reactor meltdowns and explosions triggered by the massive earthquake and tsunami of March 11.

More than seven months after the disaster, tens of thousands of people remain evacuated from their homes and businesses in a 20-kilometre (12-mile) no-go zone around the plant and in pockets beyond that.

Fully decontaminating those areas is expected to take decades.