KANSAS CITY, Missouri (Reuters) - A fire triggered by a natural gas explosion engulfed a restaurant in Kansas City, Missouri, on Tuesday, injuring at least 16 people and leaving search teams to comb charred debris for remains of anyone who may have perished, authorities said.

Although no known fatalities were immediately reported, Fire Chief Paul Berardi said several patrons were likely inside the restaurant when it went up in flames, and cadaver dogs were brought in to help sift through the ruins of the building overnight.

"We are concerned that there may be additional people that were not able to get out of the structure," Berardi told reporters, adding that the blaze erupted at happy hour. "I always fear there may be fatalities in a situation like this."

He said the results of the search with dogs would not be known "for some time."

The explosion shook the Country Club Plaza, an upscale shopping area about 30 blocks south of downtown Kansas City, around dinner time, just after 6 p.m. CST (0000 GMT), fire officials said.

Berardi said he had initial reports that 14 people were hurt, nine of whom were taken to area hospitals, including two with life-threatening injuries.

But a separate tally of patients from three hospitals showed at least 16 people were injured, three of them listed in critical condition.

"I am just keeping my fingers crossed that this turns out to be what it looks like on first blush - that this is a relatively low number of injuries compared to what it could be," Mayor Sly James told reporters.

"Hopefully, no fatalities will come out of it, but we don't know that at this point," he added.

Appearing earlier in the evening on CNN, Fire Department spokesman James Garrett attributed the explosion to a natural gas leak, and Berardi later said that the odor of gas was reported in the area before the blast.

"It does seem to be an accident," the chief said. "It doesn't appear to be foul play at this time."

The precise origin of the blast was not immediately known, officials said. However, an office building was under construction across the street from the fire scene.

One witness, Bryce McElroy, who lives about two blocks away, said he heard a loud boom and headed toward the noise, arriving on the scene to see flames leaping from a manhole cover and advancing on the restaurant, a popular steakhouse and fine dining venue called JJ's.

Jeff Hansen, who lives about four blocks away, said he went to the scene to offer assistance after hearing the blast and saw six to eight people visibly injured who were leaving the area.

"Obviously there were multiple injuries," he said. "The question is if there is anybody still in there."

Firefighters swarmed into the area to battle flames consuming the restaurant as police officers ordered bystanders to move two blocks back from the fire as a precaution. The smell of gas fumes permeated the air around the scene.

The University of Kansas Hospital received six patients from the blast and fire, two of whom drove themselves from the scene and were treated and released, hospital spokesman Bob Hallinan told Reuters.

Of the four patients transported to the hospital, one was listed in critical condition and two in serious condition, he said. The fourth was being transferred from another hospital.

Eight patients were reported at nearby St. Luke's Medical Center, two of them listed in critical conditions. Two more people were listed in good condition at Research Medical Center, spokeswoman Denise Charpentier said.

Two JJ's employees who were away from the restaurant at the time of the blast said they were told later by fellow workers who were present that staff members were alerted to the smell of gas shortly before the blast and had begun to evacuate the building.

One of those workers who relayed that account, Talley Saey, said she was told that several employees were among those taken to the hospital.