An explosion that collapsed a residential building in France’s port city of Marseille early Sunday has rescuers racing to find any victims trapped under the debris as a fire burned beneath the rubble. 

The five-story building collapsed just before 1 a.m., and six victims have been hospitalized with injuries, Marseille Mayor Benoit Payan said at a televised briefing. 

"I would like us to prepare for a difficult situation - we could have victims today," Payan said.

Hours after the collapse in France's second-largest city, it was still unclear whether anyone was killed or what caused the explosion.

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French Interior Minister Gerald Darmanin arrived at the scene later Sunday morning and told reporters that between four and 10 people were believed to be trapped under the rubble.

But the fire burning underneath the rubble complicated rescue efforts, with more than 100 firefighters working to put out blazes under the huge piles of debris while trying not to harm any potentially trapped victims.

"We’re trying to drown the fire while preserving the lives of eventual victims under the rubble," said Lionel Mathieu, commander of the Marseille fire brigade, adding that the intense heat made it impossible to send in dog teams to search.

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Residents were evacuated from two neighboring buildings that partially collapsed during the incident. Payan said one of the buildings was in danger of collapsing.

The mayor said the explosion was the "probable" cause of the building collapse, although he stressed that an investigation would ultimately determine the cause.

The explosion could be heard throughout the old quarter in the center of France’s second-largest city and resounded in other neighborhoods.

Nearby streets were blocked off as rescue efforts continued.

The Associated Press contributed to this report.

Twitter removed the "state-affiliated media" label it placed on National Public Radio's account and replaced it with a label that reads, "government funded media."

NPR was initially slapped with the "state-affiliated media" designation on Tuesday before Twitter faced public pressure to pull the label. Other media outlets with the label include Russian state-owned TASS, Russian state-controlled RT and China's official state news agency Xinhua. 

No tweets have been shared from NPR's official account since Twitter added the "state-affiliated media" label, but the news organization's President and CEO John Lansing released a statement condemning the initial designation.

"We were disturbed to see last night that Twitter has labeled NPR as ‘state-affiliated media,’ a description that, per Twitter’s own guidelines, does not apply to NPR," Lansing wrote on Wednesday. "NPR and our member stations are supported by millions of listeners who depend on us for the independent, fact-based journalism we provide. It is unacceptable for Twitter to label us this way. A vigorous, vibrant free press is essential to the health of our democracy."

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After receiving the label, NPR updated its Twitter bio to read, "NPR is an independent news organization committed to informing the public about the world around us. You can find us every other place you read the news."

White House press secretary Karine Jean-Pierre also defended NPR against Twitter's designation after a reporter asked her about it in the briefing room on Wednesday.

"Social media companies make their own independent decisions about content rules, so I won't comment on Twitter's rules, but what I will say, more broadly, I'll say there's no doubt of the independence of NPR's journalists," Jean-Pierre said.

"If you've ever been on the receiving end of their questions you know that they have their independence in journalism. NPR journalists work digitally to hold public officials accountable and inform the American people." She continued. "The hard-hitting independence nature of their coverage speaks for itself. And so, I'll leave it there."

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Twitter CEO Elon Musk appeared to endorse the "state-affiliated media" designation for NPR, writing "Seems accurate" in a tweet on Wednesday in response to the label.

The platform's policy defines "state-affiliated media" as outlets where "the state exercises control over editorial content through financial resources, direct or indirect political pressures, and/or control over production and distribution."

But Twitter's Help Center explained that "[s]tate-financed media organizations with editorial independence, like the BBC in the UK for example, are not defined as state-affiliated media for the purposes of this policy."

NPR correspondent David Gura tweeted on Wednesday that Twitter's policy was changed to remove any mention of the news outlet.

The Help Center's information has since been updated to again include NPR as a "[s]tate-financed" media organization along with the BBC.

Musk acknowledged on Thursday in an exchange with an NPR reporter that the NPR's label may not have been accurate.

"The operating principle at new Twitter is simply fair and equal treatment, so if we label non-US accounts as govt, then we should do the same for US, but it sounds like that might not be accurate here," Musk said in an email, according to NPR.

The "government funded media" label now also appears on the Twitter account of PBS.