Radioactive substances were believed to have sunk 10 cm to 30 cm into the ground after escaping from damaged Japanese nuclear reactors, Kyodo News reported, citing a research institution.

Researchers said radioactivity was detected 5 cm beneath the ground three months after the start of the nuclear emergency at the Fukushima Daiichi Nuclear Power Station, which was struck by earthquake and tsunami on March 11, 2011.

After fires and blasts, the plant released massive amounts of radioactivity into the environment.

The Japan Atomic Energy Agency said more recent estimates indicated it had seeped deeper into the ground, possibly washed down by rain, Kyodo reported.

“Further delay in decontamination works will make the radioactive materials sink deeper into the ground, and it will impose more burdens on those involved in the decontamination,” Haruo Sato, a researcher at the agency's Horonobe Underground Research Center in Hokkaido, was quoted by Kyodo as saying.

In some areas of Fukushima prefecture, local officials and concerned parents have already removed the topsoil of school playgrounds.

But government-led decontamination has barely started in areas around the nuclear plant, including the no-go zone within a 20-km radius of the facility.

The Japanese government has been criticized for awarding the first decontamination contracts to major construction companies that had benefited from building nuclear power plants.

The decontamination projects require the government to find sites to store radiation-contaminated soil and other nuclear waste, but Tokyo has already had difficulty in doing so.