Some 27 people were in cardiopulmonary arrest, an official at Osaka's city fire department told Reuters, the term used in Japan before a death is officially confirmed. Another person was injured, the official said.

Nine people were confirmed dead at hospital, broadcaster TV Asahi said.

Police were investigating suspected arson, including reports that a man started the fire in the building, Kyodo News said, citing people involved in the investigation.

An elderly man brought a bag into the building that leaked flammable liquid and was ignited, the Yomiuri newspaper said. The man was believed to be a patient at the clinic, the Mainichi newspaper said.

The fire broke out shortly after the clinic, on the fourth floor of a multi-storey office building, opened for business at 10 a.m. (0100 GMT) and was largely extinguished within 30 minutes, public broadcaster NHK reported.

Video from NHK showed smoke pouring out of the fourth-floor windows, and the roof. Footage later showed smouldering windows, blackened and charred.

"When I looked outside I saw orange flames in the fourth-floor window of the building. A woman was waving her hands for help from the sixth floor window," a 36-year-old woman who works at a company nearby told Kyodo.

Located in a shopping and entertainment district not far from Osaka's main train station, the building also houses a beauty salon, a clothes shop and an English-language school, NHK said.

Another woman who said she saw smoke coming from the window told Kyodo that power briefly went out in the surrounding area.

An arson attack at an animation studio in the city of Kyoto in 2019 killed more than 30 people and injured dozens.