Dzhokhar Tsarnaev has been sentenced to die by lethal injection for the Boston Marathon bombings.

The federal jury in Massachusetts reached the decision on Friday after more than 14 hours of deliberations over three days.

The seven women and five men were asked to determine whether Tsarnaev should be put to death by lethal injection or spend the rest of his life in prison without the possibility of parole.

The 21-year-old was convicted last month on all 30 federal counts he faced in connection with the 15 April 2013 attacks that killed three people and injured 264 others.

He was also found guilty of the fatal shooting of an Massachusetts Institute of Technology police officer.

In court for the verdict were Bill and Denise Richard, parents of eight-year-old Martin Richard, the youngest victim to die in the attack.

They had called for Tsarnaev to be sentenced to life in prison to spare their family years of likely legal challenges by the bomber.

In the penalty phase of his trial, jurors heard 11 days of testimony from more than 60 witnesses, most of them called by the defence.

His defence depicted him as a "good kid" who was led down the path to terrorism by his radicalised older brother.

But prosecutors said he was so heartless he placed a bomb behind children.

They urged the panel to go for the death penalty, portraying Tsarnaev as a remorseless terrorist who wanted to punish America.

The elder Tsarnaev, 26, was pronounced dead at the scene of a gun battle with police after his younger brother inadvertently ran him over with a car.

Jurors needed only sentence Tsarnaev to death on one of the 17 counts that carry capital punishment for him to face the possibility of execution.

The death penalty is unpopular in Massachusetts, where it is not allowed under state law. It was permitted as an option in Tsarnaev's case because he faced federal charges.