A powerful gas explosion damaged a building in Paris' Left Bank on Wednesday, injuring more than two dozen people and sparking a large fire, authorities said. 

There were a total of 37 people injured, French Interior Minister Gérald Darmanin said on France's national BFM TV network. Four people were in "absolute emergency" and 33 were "relatively injured." 

Two people are still missing, and the rubble of the damaged building is being searched, Darmanin said. There were 320 firefighters and 200 policemen on site. 

"We still don't know the origin of the explosion," Darmanin said. 

Police and a national government minister quickly urged people to avoid the area. 

Fire officials did not immediately confirm the district mayor's statement that the explosion was caused by a gas leak, but witnesses reported smelling gas in the area before the blast.

Witnesses told BFM and other outlets the blast had seriously damaged a building housing the Paris American Academy language school. BFM said it was unclear whether the initial blast had struck the academy or a neighboring building. Officials told CBS News that the blast had "weakened" neighboring buildings. 

"Because it was a Wednesday afternoon, the children were not in class, which most probably avoided more casualties," Darmanin said on BFM.

About 20 families who lived in the building or in two neighboring buildings will be rehoused, officials told Le Parisien, a French daily newspaper. 

French news outlets said multiple buildings in the vicinity caught on fire after the explosion. Images from the scene showed firefighters appearing to have control of the blaze.

The Paris public prosecutor's office and local services have mobilized victim support associations, officials said. 

Laure Beccuau, a Paris public prosecutor, told the Agence France-Presse that a legal investigation into the explosion was underway. 

"An investigation is now underway, as part of legal investigation that has been opened on the following charges: unintentional injury with the aggravating circumstance of deliberately endangering the lives of others," Beccuau said. "I would stress that this charge was chosen because we felt it was the most appropriate on the basis of the evidence available at the time. There is nothing to prevent this classification from evolving, even if we have initial elements that lead us to confirm that this explosion originated in the building."

The U.S. State Department was not aware of any American citizens injured or killed in the explosion, a State Department spokesperson said. 

The American embassy in France issued a security alert advising U.S. citizens to avoid the area. 

While speaking at an annual music festival in Paris, President Emmanuel Macron asked for a moment to think of "those who were injured" in the explosion, and the families of the victims. 

"I want to say a word for all the victims, the families, who live in anguish and difficulty at a time when the toll is not stabilized," he said. 

This is a developing story. Please check back for updates.

In:
• Paris
• Fire
• Explosion
• Gas Leak

Tucker Reals

Tucker Reals is the CBSNews.com foreign editor, based at the CBS News London bureau.