Elon Musk has questioned the mayor of San Francisco’s “priorities” as he alleged the city is more concerned about Twitter staff sleeping in the office than about children suffering accidental overdoses.

The Twitter owner’s outburst followed controversy about bedrooms appearing at the social media site’s office in the northwest California city.

“Sad little conference-room sleeping quarters” have been installed, Forbes magazine reported.

The accommodation features “unmade mattresses” and “drab curtains”, it added.

Its appearance is perhaps to be expected after Musk told Twitter staff they needed to work “long hours at high intensity”.

“Going forward, to build a breakthrough Twitter 2.0 and succeed in an increasingly competitive world, we will need to be extremely hardcore,” he told them.

“This will mean long hours at high intensity. Only exceptional performance will constitute a passing grade.”

Last month, Twitter staffer Esther Crawford said: “When your team is pushing round the clock to make deadlines, sometimes you sleep where you work.”

Musk appeared to confirm the bedrooms’ existence after local media picked up the story.

He tweeted: “So city of SF attacks companies providing beds for tired employees instead of making sure kids are safe from fentanyl. Where are your priorities Mayor London Breed!?”

He attached an article from the San Francisco Chronicle which said a 10-month-old baby had suffered an accidental fentanyl overdose at a playground.

Paramedics administered overdose-reversing medication and the youngster was discharged, the paper added.

Ms Breed’s spokesperson, Parisa Safarzadeh, said the mayor’s office was investigating, but “thankfully a fatal incident was avoided thanks to the quick response of (emergency services)”, the newspaper reported.

San Francisco’s Department of Building Inspection is investigating a complaint about the bedrooms, Forbes magazine said.

It quoted Patrick Hannan, the department’s communications director, as saying: “We need to make sure the building is being used as intended.”

In a separate development, Musk briefly lost his title of richest person in the world on Wednesday, again according to Forbes.

It said he was briefly replaced by Bernard Arnault, chief executive of luxury brand Louis Vuitton’s parent company LVMH, and his family.

Musk’s short dip in status was caused by a steep drop in the value of his stake in Tesla and the $44bn he spent on Twitter.