The Japanese government said Friday that it has conducted an on-site inspection of the U.S. Yokota Air Base in western Tokyo over the possible leak of so-called PFAS chemicals that may have harmful effects on human health.

Officials from the defense, foreign and environment ministries, the Tokyo metropolitan government as well as local municipalities entered the base in the morning to inspect a firefighting training area where an extinguishing agent containing PFAS is believed to have been used, according to the Japanese Defense Ministry.

Acknowledging local concerns over the chemicals, Defense Minister Gen Nakatani told a press conference that the government will work to ensure that "environmental measures in and around U.S. military facilities in Japan will be effective."

Water sample testing of a reservoir on the firefighting training site is also being arranged under a Japan-U.S. agreement allowing local officials to enter U.S. military bases in Japan to conduct environmental surveys, according to Nakatani.

The U.S. military in Japan has already stopped using extinguishers that include PFAS, but past U.S. investigations detected levels in reservoirs far above Japan's nonbinding interim standards.

The inspection that took place on Friday was conducted at the request of nearby municipalities after the United States informed the Japanese government in October of the possible leakage of water containing PFAS from the firefighting training area following heavy rainfall in late August.

PFAS is a general term for a group of over 10,000 artificial chemicals that include PFOS, or perfluorooctanesulfonic acid, and PFOA, or perfluorooctanoic acid.

While used in a wide variety of products including foam extinguishers and frying pan coatings, PFAS are known as "forever chemicals" because they break down very slowly over time and can accumulate in people, animals, plants and the environment. Exposure to certain levels of PFAS may lead to increased risk of some cancers.