BOSTON Marathon bomber Dzhokhar Tsarnaev has been sentenced to death for his part in the lethal terror attack.

A federal jury in Massachusetts announced that Tsarnaev is to be executed by lethal injection after bringing to an end more than 14 hours of deliberations.

The 21-year-old was convicted last month of causing the death of three people, including an eight-year-old boy, during the deadly attack in 2013 which also injured more than 260 others.

Tsarnaev killed a Massachusetts Institute of Technology police officer days later. He was found guilty of all 30 federal charges against him, 17 of which carried the possibility of the death penalty.

The defence sought to save Tsarnaev's life by pinning most of the blame on his radicalised older brother, Tamerlan, who was killed during the manhunt that followed the attack.

But prosecutors portrayed the young man as an equal partner in the bombing and so heartless he placed a bomb behind children.

Tsarnaev showed no reaction as the jury sentenced him.

His head was down slightly and his hands were folded in front of him as the sentence was handed down.

During the trial the defence emphasised the young age of the terrorist, who was 19 at the time of the attacks.

Lawyers also highlighted his difficult early life. The Tsarnaevs - ethnic Chechens - had lived in the former Soviet republic of Kyrgyzstan and the volatile Dagestan region of Russia, near Chechnya. The family moved to the US in 2002.

After the sentence was announced, US Attorney General Loretta Lynch said: "The ultimate penalty is a fitting punishment for this horrific crime and we hope that the completion of this prosecution will bring some measure of closure to the victims and their families.''

The three victims of the terror attack were Restaurant manager Krystle Campbell, 29, who had gone to watch a friend complete the race, Chinese graduate student Lu Lingzi and Eight-year-old Martin Richard, who was standing with his family, cheering the runners.

The boy's father, Bill, testified during the trial telling how he had to leave his terminally injured son behind and rush his seven-year-old daughter Jane, who lost her leg in the blast, to hospital. He did not see Martin alive again.

Bill Richards was among those who had opposed the death sentence, saying that his family would rather have closure than see the matter dragged through the media each time an appeal is made.

In an article for the Boston Globe, he wrote: "The continued pursuit of that punishment could bring years of appeals and prolong reliving the most painful day of our lives.

"We hope our two remaining children do not have to grow up with the lingering, painful reminder of what the defendant took from them, which years of appeals would undoubtedly bring."

Tsarnaev is likely to be moved to a federal prison in Terre Haute, Indiana, where he will be placed on death row.

However, it is unlikely he will be executed in the near future as his defence team will now begin a lengthy appeals process.

On average, those sentenced to death in the US spend 10 years in prison before they are executed, and only three people have been put to death for federal crimes since 1988.