Extremely high levels of radiation have been discovered in a playground in Tokyo, officials say, fanning fears for the health of children in the area.

Soil underneath a slide at the park in Toshima ward in the north-west of the Japanese capital showed radiation readings of up to 480 microsieverts per hour, the local administrative office said.

Anyone directly exposed to this level would absorb in two hours the maximum dose of radiation Japan recommends in a year.

"Many children play in the park daily, so the ward office should explain the situation," Kyodo News quoted a 62-year-old local woman as saying.

Officials were made aware of the contamination after a local resident reported it on Monday and said they did not think it was connected to the disaster at Fukushima in 2011.

"Because the area in which we detect radioactivity is very limited, and readings in surrounding parts are normal, we suspect radioactive materials of some kind are buried there," local mayor Yukio Takano said in a statement.

According to a local official, the park was built in 2013 on what was previously a parking lot for Tokyo's sanitation department.

Top soil at the lot was replaced before the land was turned into a park, the Toshima official said.

The park has been sealed, and Toshima ward is discussing with experts how to approach the problem, she added.

The radiation level at the park was over 2,000 times that at which the national government required soil cleaning in areas around the Fukushima Daiichi nuclear plant, where reactors melted down after the March 2011 tsunami.

But that standard is for measurements taken at 50 centimetres to one metre above ground, while officials in Toshima ward checked the ground itself.

Many families in eastern Japan continue to survey the levels of radioactive contamination around their houses, distrustful of government assurances that most places were not affected by the Fukushima meltdown.

Such efforts have led some people to discover radioactive materials that have been dumped in their neighbourhoods.

Months after the Fukushima crisis started in 2011, officials found bottles of radium, used for medical tools and a glow-in-the-dark paint for watches, discarded under a private house and a supermarket in Tokyo.

The playground incident came days after a small drone showing traces of radioactivity was found on the roof of the prime minister's official residence.

AFP