Four workers in their 50s died after falling into a manhole during a sewage inspection in Gyoda, north of Tokyo, Al Jazeera reported.

One worker reportedly slipped into the 10-metre-deep shaft, and three others fell while trying to rescue him.

Emergency crews detected toxic hydrogen sulfide gas at the scene. All four were retrieved and later pronounced dead in hospital, according to Al Jazeera.

Authorities say the cause is still under investigation.