TOKYO  Fri Apr 24, 2015 10:09am BST

Authorities in the Japanese capital have cordoned off a playground where high levels of radiation were detected this week, reviving concerns about nuclear contamination four years after the Fukushima disaster.

Nuclear regulators measured elevated radiation levels on Thursday in a children's park in central Tokyo, city officials said, more than 250 km (155 miles) from the crippled Fukushima nuclear plant in northeast Japan.

One area of the park registered 480 microsieverts per hour, or nearly half the recommended annual limit of exposure.

That compares with radiation levels ranging from 0.7 to 23.2 microsieverts per hour measured this month in the air in Okuma, a town less than 20 km from the Fukushima plant.

It was not immediately clear why radiation levels were high in the park, which opened two years after the 2011 disaster. It has now been fenced off with an orange barricade, the city said.

"We are still checking what caused these elevated radiation levels and what type of radioactive substance it is," said park representative Kyoko Tsujimura, adding that radiation elsewhere in the park remained low.

On Friday, television broadcast images of regulatory officials in work uniforms measuring radiation from a patch of dirt beside a colorful slide in the park.

The city has set up a health advice office to answer queries from concerned parents.

Thousands of people are still living in temporary shelters and have been unable to return home to hot zones closest to Tokyo Electric Power's nuclear plant.

In the months after the nuclear meltdowns, concerned parents and citizens began measuring radiation levels in Tokyo schools and parks.

A drone with a small amount of radiation was found on Japanese Prime Minister Shinzo Abe's office building this week.