Boston Bomber Tells Victims: 'I Am Sorry'

Boston Marathon bomber Dzhokhar Tsarnaev has apologised to his victims during a death sentence hearing, his first public remarks since the 2013 attack.

"I am sorry for the lives that I've taken, for the suffering that I've caused you, for the damage that I've done. Irreparable damage," said the 21-year-old.

He struggled to maintain his composure as he spoke for five minutes at the hearing, shortly before he was formally condemned to die by lethal injection.

The former university student, who was convicted in April, said there was "no doubt" of his guilt.

He also said he was a Muslim and hoped Allah would have mercy on his victims, as well as on himself and his brother.

Tsarnaev, who wore a dark blazer, stood and faced the judge, but spoke of the victims.

"I pray to Allah to bestow his mercy upon the deceased," Tsarnaev said.

"I ask Allah to have mercy upon me, upon my brother, upon my family."

Tsarnaev and his older brother, Tamerlan, went on the run after the bombings until he was arrested and his sibling killed as police cornered them four days later.

Survivors were divided on whether his apology was genuine.

Lynn Julien said his remarks "were sort of shocking" and disputed whether he had shown true remorse.

"A sincere apology would've been nice," she told reporters.

But another survivor, Henry Borgard, welcomed Tsarnaev's statement and said he hopes he finds peace.

Federal prosecutor Carmen Ortiz said it was notable that Tsarnaev did not renounce terrorism.

In court earlier, the mother of a woman killed in the attacks told Tsarnaev: "What you did to my daughter is disgusting."

Patricia Campbell was the first person to deliver a victim-impact statement on Wednesday.

Her daughter, 29-year-old Krystle Campbell, was one of three people killed in the blasts.

She was among many survivors and relatives who spoke at the hearing.

The father of the youngest person killed in the blasts, eight-year-old Martin Richard, told Tsarnaev he did not have to place his bomb behind a group of children.

Bill Richard, joined by his wife Denise, said Tsarnaev instead "chose to accompany his brother and participate in this hate".

Krystle Campbell's best friend, Karen Rand McWatters, who was one of 17 people to lose a leg in the blasts, told Tsarnaev he must not have a soul.

She said she could not understand why he "chose to leave his weapon of mass destruction behind those children as he walked away".

The sister of Sean Collier, a police officer shot dead by Tsarnaev and his brother after the bombings, also spoke in court.

Jennifer Rogers said: "He (Tsarnaev) is a coward and a liar. He bought milk after setting off a bomb to kill children."

A jury sentenced Tsarnaev last month to die for the April 2013 bombing, which also injured 264 people.

Tsarnaev's lawyers are expected to file an appeal, and the legal wrangling could drag on for many years.