Prime Minister Shinzo Abe called the fire in the city of Kyoto - the latest grisly killing in a country known for its low crime rates - "too appalling for words" on Twitter and offered condolences to the victims.

Police had taken a 41-year-old man into custody who had shouted "Die" as he poured what appeared to be petrol around the three-story Kyoto Animation building shortly after 10 a.m. (01:00 GMT), public broadcaster NHK reported.

White and black smoke billowed from the windows of the building, television footage showed.

The studio produces popular series such as the "Sound! Euphonium". Its "Free! Road to the World - The Dream" movie is due for release this month.

"I heard the sound of fire engines and stepped outside my house, and saw big flames spewing out of the building," NHK quoted a 16-year-old boy as saying.

"Fire department officials were trying to rescue the injured in a nearby park but it seemed like there weren't enough of them," he said.

The casualties included seven confirmed dead, six with no vital signs, and another 10 or so people found unconscious, a Kyoto city fire department official told Reuters.

Another 36 have been injured, 10 of them seriously, the official said.

Police can yet question the suspect as the arsonist was being treated in hospital for his injury, said NHK.

The fire department official said earlier that some 30 people, who were believed to have been in the building at the time of the fire, could not be reached.

Some or all of the 10 or so people later found unconscious on the top floor and the staircase leading to the roof may be included in that tally, he said.

Kyoto police declined to comment.

Violent crime is relatively rare in Japan but occasional high-profile incidents have shocked the country.

REUTERS