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Three teenagers in Finland have been handed prison sentences over the brutal murder of a 16-year-old boy last year.

The defendants, who were all born in 2004 and knew the victim, received sentences of 10, nine and eight years at a court in the capital Helsinki.

The case is seen as highly unusual in Finland, because of the age of the attackers and the nature of the crime.

Police had described the prolonged and fatal assault on the young victim last December as "brutal".

All three defendants had previously admitted to assaulting the victim, but had all denied murder.

On the night of the incident, the teenagers had been celebrating a 16th birthday and had reportedly all consumed alcohol.

The defence argued that they had not intended to kill the boy, and that they did not believe he would die as a result of their actions. They argued that the defendants should serve no more than four years in prison.

Prosecutors, who were seeking prison terms of at least 12 years, said the victim had been physically assaulted for up to four hours before his death on Friday 4 December in the Koskela district of Helsinki.

They said the boys had planned in advance to ply their victim with alcohol. The victim is believed to have been left at the scene unconscious and none of his attackers alerted authorities throughout the weekend.

He was not discovered until the following Monday by construction workers not far from a hospital, police said.

The arrests were made after the mother of one of the three boys contacted the police.

The oldest defendant, who the court said had been the most violent, received the longest sentence of 10 years and one month in prison.

The second-youngest was sentenced to nine years and two months, while the third and youngest defendant received a jail sentence of eight years and two months. All three have the right to appeal against their sentences.

Marko Forss, detective chief inspector of the Helsinki police department, described the assault at the time as "particularly brutal and cruel".

"The victim was subjected to severe and prolonged violence inflicted on various parts of the body," he said, adding that all three defendants had left the area "presumably... aware of the victim's condition".

The three teenagers were also accused of committing an act of robbery in relation to the assault, with a fourth teenager facing charges over that incident.

While minors cannot be sentenced to life in prison under Finnish law, crimes committed prior to a murder can also contribute to sentencing, meaning the boys faced jail terms of up to 15 years.

Offenders aged between 15 and 17 when a crime is committed come under Finland's Young Offenders Act, which limits jail sentences to between six months and four years in a juvenile prison unless there are exceptional circumstances.

"A long prison sentence is regarded as having negative effects on such a young person," Raimo Lahti, a professor of criminal law at the University of Helsinki, told the ´óÏó´«Ã½.

In most cases the law for young offenders was designed to focus on special prevention, he said, with the emphasis on supervision after their release.