Kansas City official Troy Schulte said the authorities did not know if anyone was killed in the fire, which destroyed a restaurant and other businesses.
Local TV showed flames shooting out of the Country Club Plaza retail area.
Troy Schulte said officials believed the explosion may have stemmed from a utility contractor accident.
A doctor with St Luke’s Hospital told reporters two of the injured being treated there were in critical condition, one with “pretty severe burns”.
Six others walked into the hospital with minor injuries.
The Kansas City Star reported a total of 15 patients across three area hospitals, including St Luke’s Hospital.
A manager of the restaurant destroyed in the blaze, JJ’s, told police three people were unaccounted for but it was unclear if they had left earlier.
Sniffer dogs were being used to search the smoldering wreckage for bodies, city officials said.
First images from the scene showed firefighters and other emergency teams battling a massive blaze that appeared to have engulfed an entire block.
Jill Chadwick, spokeswoman for the University of Kansas Hospital, said one patient had told her that the restaurant was being evacuated when the blast happened and the roof fell in.
“This patient told me there had been the smell of gas and that they had evacuated any patrons and that employees of the restaurant were in the process of shutting off gas valves and trying to get out of the restaurant when there was the explosion,” she said.
“He said the last thing he remembered was the roof collapsing. I asked him how he got out and he said the front of the restaurant was blown away and he just walked through the rubble.”
The initial blast was felt beyond the shopping area.
The shopping area, established in 1922, is based on the architecture of Seville, Spain, and includes retail, restaurants, apartments and offices.
There were signs saying that utility work was being done in area ahead of the blast, and one man told the Star he saw Missouri Gas Energy workers inside JJs with gas detectors.
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