Boston Marathon bombing: Prosecutors to argue for Tsarnaev's execution

US prosecutors will today begin laying out the arguments they hope will convince a jury to sentence Dzhokhar Tsarnaev to death for his part in the Boston Marathon bombing.

The 21-year-old was convicted earlier this month of carrying out the 2013 terror attack alongside his older brother, who died in a shoot-out with police days after the bombing.

Lawyers and jurors will return to the same Boston courtroom this morning for the start of the sentencing phase of the trial, which will determine whether Tsarnaev should face death by lethal injection or life in prison.

Both prosecution and defence will call new witnesses and make fresh arguments to the twelve jurors, who will ultimately decide Tsarnaev's fate.

The jury must vote unanimously to sentence Tsarnaev to die, meaning that any one of the twelve jurors can veto the death penalty. The sentencing phase is expected to last several weeks.

Defence lawyers trying to save Tsarnaev's life will paint him as an impressionable teenager who was in thrall to his domineering older brother Tamerlan, who they argue was the mastermind of the attack.

Tsarnaev's lawyers are expected to cast him as an unsettled young man from a chaotic family who never fully adjusted to life in the US after arriving as a child from his native Kyrgyzstan.