Boston Marathon bomber Dzhokhar Tsarnaev has apologised for the deadly 2013 attack for the first time.

At a hearing to formally sentence him to death, Tsarnaev, 21, said: "I am sorry for the lives I have taken, for the suffering that I have caused you, for the damage I have done, irreparable damage.

"In case there is any doubt, I am guilty of this attack, along with my brother."

The scene near the Boston Marathon finish line as a bomb detonated. Credit: Reuters

The parents of the victims were in the packed courtroom to hear the 21-year-old ethnic Chechen address the court for the first time.

Tsarnaev was found guilty of killing three people and injuring 264 people in the bombing on April 15th 2013, as well as shooting dead a police officer three days later.

The attack on the world-renowned race was one of the most high-profile attacks on the US since 9/11.

Tsarnaev's brother Tamerlan was killed by police in a shootout shortly after the deadly attack.