According to Osaka city fire department spokesman Akira Kishimoto, the fire began on the fourth level of an eight-story building in Osaka's Kitashinchi shopping and entertainment district.
Mr. Kishimoto said the fire affected 28 people, 27 of whom were found in cardiac arrest - a word used in Japan to refer to someone who has died but has not been officially certified. One person sustained injuries.
He stated that 23 patients had been transported to local hospitals thus far.
According to NHK national television, the building houses an internal medicine clinic, an English language school, and other companies.
The cause of the fire and other data are unknown at this time.
Police in Osaka said they were investigating whether the fire was started intentionally or by accident.
NHK footage showed dozens of fire engines and police cars parked near the building, with people across the street watching the unfolding events.
According to NHK, a witness heard a woman's voice pleading for assistance from the fourth story.
Mr. Kishimoto stated that residents on the building's remaining floors were thought to have been evacuated.
Another witness told TV Asahi that when he came outside after hearing a commotion, he noticed orange flames and grey smoke spewing out of windows on the fourth story of the building.
Seventy fire engines were dispatched to battle the blaze, which was doused mainly within about 30 minutes following an emergency call, officials said.