Washington: The US federal judge in Boston formally sentenced 21-year-old Dzhokhar Tsarnaev to death for his role in the 2013 Boston marathon bombing attacks and the killing of a police officer when on the run, media reported on Thursday.

Before the court judge read out the ruling, Tsarnaev broke a two-year silence apologising to victims for the bombings he and his brother Tamerlan Tsarnaev unleashed at the finish line of the marathon in April 2013, which killed three people and injured 260, Xinhua news agency reported.