Many victims of an arson attack on a Japanese animation studio were young with bright futures, some joining only in April, the shaken company president said, as the death toll climbed to 34.

Thursday's attack on Kyoto Animation, well known for its television series and movies, was Japan's worst mass killing in two decades.

Many of the victims of the attack, in the ancient capital of Kyoto, were young women, company president Hideaki Hatta said.

"Some of them joined us just in April. And on the eighth of July, I gave them a small, but their first, bonus," he said.

"People who had a promising future lost their lives. I don't know what to say. Rather than feeling anger, I just don't have words," Mr Hatta said.

Fifteen of the victims were in their 20s and 11 were in their 30s, public broadcaster NHK said. Six were in their 40s and one was at least 60.

The studio had about 160 employees.

Police have confirmed the identity of the suspect as Shinji Aoba, but have declined to comment further.

Aoba had been convicted of robbery in the past and is suspected of carrying out the arson attack because he believed his novel had been plagiarised, Japanese media have said.

But Mr Hatta said he had no idea about any plagiarism claim, adding he had not seen any correspondence from the suspect.

Police have not formally arrested Aoba, as he is being treated for heavy burns, NHK said, but have taken the unusual step of releasing his name.

Near the blackened studio building, animation fans gathered to add to a growing pile of flowers, drinks and other offerings.

Police guarded the site as investigators, some on the roof near where many died in a connecting stairwell, examined the blackened building.

Kyoto Animation produces popular "anime" series such as the "Sound! Euphonium".

It is also known for "Violet Evergarden", which has been shown on Netflix.