A 16-year-old kickboxer allegedly asked to meet Artyom Shustov, 19, and the pair got into a furious row in a park.
The teen accused the older teenager of forcing himself on his girlfriend, 17, at a party.
Shustov insisted the girlfriend had "started pestering him at the party, and he was drunk and could not resist her", according to Russian newspapers citing law enforcement sources and witnesses.
The teen allegedly stabbed Shustov , 19 (pictured) twice with a kitchen knife
The pair began fighting and the 16-year-old took out a kitchen knife from his backpack and twice stabbed his rival in the back and abdomen, it is claimed.“He put some stones in the backpack with the head and threw it to the River Silinka”Newspaper DVnovosti
With Shustov still alive but badly wounded, the 16-year-old allegedly "started to cut off his head with the knife".
He then "took the severed head by the hair, put it in a garbage bag, and then into a backpack", reported newspaper DVnovosti in Komsomolsk-on-Amur.
"He dragged the body to another part of the park and put Artyom's student papers on his chest."
"Then he went home, hid his bloodstained clothes in his wardrobe, took a shower and went to bed," reported The Siberian Times.
"The next day [the 16-year-old] went to [his girlfriend] and told her about the murder and showed her the severed head."
The 16-year-old "wanted to bring the head to his [girlfriend] to prove his love in this way", according to police who say the teenager later confessed to the beheading in a formal statement.
Police have interviewed the "distraught" girl, but no further details have been given.
She has not commented on the grisly killing.
After showing her the severed head, the 16-year-old tried to hide it, police sources claim.
"He put some stones in the backpack with the head and threw it to the River Silinka," reported DVnovosti.
The girl had posted a month before the killing an image of her and Shustov together, possibly at the party which led to the scandal.
The picture is spooky since it only shows his head as she makes a shocked expression.
Friends have told police that the girl had admitted to the accused that she had oral sex with Shustov, but claimed he had forced himself on her.
Shustov was still alive when the teenager 'started to cut off his head with the kn...
"We were then in the apartment with them and personally saw what was happening," said one friend.
"The next day, Olena obviously regretted what had happened and she her boyfriend. Yet she explained it as if Artyom had harassed her and tried to rape her," reported the local newspaper.
"[The 16-year-old] was shocked and asked Artyom to meet and talk.
"They knew each other before. Artyom agreed and they arranged their meeting at the entrance of the park."
Shustov arrived with a female friend but she left when the pair started to argue.
Witnesses at Railway Workers Park in the city of 260,000 people in the Russian Far East said that during the argument Shustov had tried to flee when he saw the 16-year-old had a knife.
The 16-year-old was detained and police confirmed his confession.
A murder probe is underway, confirmed the Investigative Committee.
"We were then in the apartment with them and personally saw what was happening," said one friend.
"The next day, Olena obviously regretted what had happened and she her boyfriend. Yet she explained it as if Artyom had harassed her and tried to rape her," reported the local newspaper.
"[The 16-year-old] was shocked and asked Artyom to meet and talk.
"They knew each other before. Artyom agreed and they arranged their meeting at the entrance of the park."
Shustov arrived with a female friend but she left when the pair started to argue.
Witnesses at Railway Workers Park in the city of 260,000 people in the Russian Far East said that during the argument Shustov had tried to flee when he saw the 16-year-old had a knife.
The 16-year-old was detained and police confirmed his confession.
A murder probe is underway, confirmed the Investigative Committee.
