NEW YORK (AFP) - Boston bomber Dzhokhar Tsarnaev, a 21-year-old naturalised American of Chechen descent, will be formally sentenced to death in court on June 24, a US judge said Thursday.

Judge George O'Toole's sentencing comes after a jury decided unanimously that Tsarnaev should die for his role in the 2013 Boston Marathon attacks that killed three people and wounded 264 others.

Tsarnaev will be given a chance to speak and victims are also entitled to make impact statements.

He will then be flown either to America's only "super-max" prison, ADX Florence, in Colorado or to the penitentiary at Terre Haute, Indiana, where male inmates sit on federal death row.

A jury took more than 14 hours to choose death rather than life imprisonment for Tsarnaev.

Government prosecutors delivered a powerful closing argument, calling him a remorseless terrorist who deserved to die and declaring that life imprisonment would be the "minimum" punishment.

The verdict handed a stinging defeat to the defence, who argued for a "lost kid" who would never have committed such horrors without being manipulated by his older brother.

The decision, possible under federal law, sparked some controversy in the state of Massachusetts, which outlawed capital punishment in 1947 and where polls suggested many residents favored a life sentence for the convicted bomber.