Prosecutors on Thursday demanded a 12-year prison term for a man accused of physically abusing and killing his partner's 1-year-old son and injuring her 3-year-old daughter in Izumisano, Osaka Prefecture, in March.

Prosecutors told the Osaka District Court that Masahiko Shibata, 20, "has not a bit of humanity, as he treated small children like toys and abused them for fun. He doesn't seem sorry, either."

Shibata's lawyers sought leniency, arguing that he did not cause any injury that led to the boy's death. They said he has also shown remorse.

The court will deliver a verdict Sept. 9.

According to the indictment, Shibata and his partner, Kotomi Manda, 24, frequently beat Manda's son from early March to March 14. The baby died March 16 in their Izumisano apartment.

The couple also allegedly used lighted cigarettes to burn the back of Manda's second daughter. Her other daughter is 5.