An apparent gas leak caused a Manhattan apartment building to explode, injuring 20 people, four of them children who were critically burned, officials said.

The explosion blew out some of the 20-unit building's walls just after 4 p.m. in upper Manhattan's Harlem neighborhood, witnesses said. Residents said they heard the explosion blocks away, then screams.

"It does appear to be a gas leak explosion that then causes a flash fire," said Fire Commission Nicholas Scoppetta.

The injured included five people with serious burns, including the children, officials said. The others injured were in stable condition, fire officials said.

A firefighter was hurt by falling debris when he entered the first floor of the five-story building, Scoppetta said.

In the aftermath, firefighters tossed pieces of drywall in a pile outside the building, while the injured were treated in stretchers on the street.

The local utility, Consolidated Edison, had no immediate evidence of a leak, but investigators were waiting until the building was stabilized before going into the apartment where the explosion occurred, spokeswoman D. Joy Faber said.

Some residents said one apartment was operating as an illegal restaurant, cooking and selling takeout food to taxi drivers. City officials said they had no evidence of an illegal business in the building.

The building and those on both sides of it were evacuated.