TOKYO — Thirty people were feared dead in an arson attack on a Japanese animation studio on Thursday, authorities said, after a man was seen shouting "die" as he doused the building with fuel in the nation's worst mass killing in nearly two decades.

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' families.

Police arrested a 41-year-old man who had shouted "die" as he poured what appeared to be petrol around the three-story Kyoto Animation building shortly after 10 a.m. (0100 GMT), public broadcaster NHK reported.

Twenty people were confirmed dead and about 10 were showing no vital signs, an official for the Kyoto City Fire Department said, a term authorities often use before a death has been confirmed by a doctor.

White and black smoke billowed from the building's charred windows, television footage showed.

It was Japan's worst mass killing since a suspected arson attack on a Tokyo building in 2001.

"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 prime minister said the cause was arson.

"Today, many people were killed and wounded in an arson murder case in Kyoto," Abe said in a post on Twitter.

"It is too appalling for words."

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.

The 10 people without vital sounds were found in the studio, some on third floor and others in a staircase leading up to the roof, the fire official said.

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

The suspected arsonist was injured and was being treated in hospital, so police could not question him, NHK said.

Kyoto police declined to comment.

There was an outpouring of support for the studio on Japanese-language social media, with some users posting pictures of animation. Many posted with the hashtag "#PrayForKyoani" — using an abbreviation for Kyoto Animation.

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

Less than two months ago, a knife-wielding man slashed at a group of schoolgirls at a bus stop in Kawasaki, just south of Tokyo, killing one girl and the father of another, while injuring more than a dozen children.

In 2016, a man armed with a knife broke into a facility for the disabled in a small town near Tokyo and killed 19 patients. — Reuters