Contractors might be illegally dumping radiation-tainted soil, vegetation and water into rivers or other places near the stricken Fukushima No. 1 nuclear plant, the Environment Ministry said Friday.

The ministry plans to summon senior officials from the contractors to the Fukushima Office for Environmental Restoration to answer questions on how they manage contaminated waste.

Some of the contractors hired to decontaminate areas tainted by the fallout are suspected of illegally dumping tainted material in the coastal town of Naraha, the hard-hit village of Iitate, and even farther inland in the city of Tamura, the ministry said.

A special law made due to the nuclear disaster bans illegal dumping of contaminated substances into the environment and makes it punishable by up to five years in prison or a fine of up to ¥10 million. The guidelines for decontamination work require contractors to store tainted substances at the decontamination site or at temporary storage sites.

"It is very regrettable if that is true," Fukushima Gov. Yuhei Sato said of the suspected dumping at his first news conference of 2013.