Four workers have died in Japan after falling into a manhole near Tokyo as they inspected sewage pipes, according to public broadcaster NHK, quoting police.

NHK reported on Sunday that the incident in the city of Gyoda in Saitama Prefecture, north of the Japanese capital, happened on Saturday, as the four men, all in their 50s, and other co-workers were inspecting a sewage pipe.

City officials say the workers were conducting an emergency inspection of sewage pipes that the central government had ordered municipalities to carry out, after a huge road cave-in in January.

Police were quoted by NHK as saying that during the inspection, one of the workers fell down the manhole, followed by three of his co-workers who were trying to save him.

According to police, the manhole is 60cm (24in) in diameter and more than 10 metres (33ft) in depth.

The fire department from the area also confirmed the incident to the AFP news agency.

Video clips published by NHK showed several emergency and rescue personnel near the manhole.

The department said rescuers detected hydrogen sulfide ? a gas toxic in high concentrations ? coming out of the manhole.

But city officials refused to be drawn on the cause of the initial fall.

?Detailed circumstances leading up to the accident are still unknown, so it?s too early for us to say anything about our responsibility,? a Gyoda city official told AFP, on condition of anonymity.

The four workers were retrieved and taken to hospital, where they were pronounced dead, according to local media reports.

About 10 workers were at the scene of the inspection, ordered to clean the pipes of wastewater and sludge if necessary. (Al Jazeera)

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