Japanese authorities on Friday staged an inspection of a US military base in Tokyo, a government spokesman said, after being informed by the American side of a chemical leak.

Japan’s probe at the Yokota Air Base followed a US notice two months ago that water containing PFOS — classified by the World Health Organisation as “possibly carcinogenic to humans” — had spilled from the site.

PFOS is part of a large group of man-made chemicals known as PFAS, sometimes called “forever chemicals” because they do not degrade easily, experts say.

The US military informed Tokyo in October that the PFOS-laced water had leaked from an area of the base where a fire-fighting drill was being carried out, Fumitoshi Sato, deputy chief cabinet secretary, told reporters.

“This inspection was realised in response to the fears and concerns harboured by local residents, and we will continue to work together with the US side,” Sato said.

Officials including from the defence ministry and Tokyo’s metropolitan government visited the site on Friday, he said. Yokota Air Base was not immediately available for comment.

America’s military presence in Japan has frequently stoked local discontent in the past, with everything from noise to pollution to helicopter accidents.

This frustration is perhaps most evident on the southern island of Okinawa, which despite comprising just 0.6 per cent of Japan’s landmass, hosts the vast majority of the country’s US military bases.

Okinawa is located east of Taiwan, a flashpoint for tensions between the US and China.

Earlier this month, the US began relocating thousands of Marines from Okinawa, with an initial “detachment of approximately 100 logistics support Marines” transferred to the US island territory of Guam.