Japan's eastern Chiba and Saitama prefectures, which both neighbor Tokyo, cut water supply to part of their households on Friday and Saturday after hazardous formaldehyde was found in it, local media reported on Saturday.

Saitama Prefecture stopped water intake and supply at one of its filtration plants after higher than permitted levels of formaldehyde was detected in tap water there, while neighboring Chiba has halted water intake at three plants for the same reason, Kyodo News reported.

The plants involved were taking water from the Tone River, which flows through the nation's densely-populated Kanto region into the Pacific, and one of its branches, local governments said.

Local authorities have been checking the water quality at the head of the river in Gunma prefecture. The source of the contamination has not been determined.

Formaldehyde is a cancer-causing chemical that is colorless and pungent. Local governments said the incident will not pose a risk to human health, according to Kyodo.