BOSTON: Boston bomber Dzhokhar Tsarnaev on Wednesday apologised in court for the first time to his victims for the suffering he caused, moments before being formally sentenced to death by a federal judge.

The US citizen of Chechen descent was sentenced to death on six counts over the 2013 bombings, one of the worst assaults on US soil since the September 11, 2001 attacks.

“I would like to now apologise to the victims and to the survivors,” said the 21-year-old former university student in his first public remarks since the April 15, 2013 bombings that killed three people.

“I am guilty,” he said, standing pale and thin in a dark blazer. “Let there be no doubt about that.”

Tsarnaev said he listened throughout the 12-week trial as he learnt about the victims from often harrowing testimony.

“I am sorry for the lives I have taken, for the suffering, the damage that I have done,” he said, beginning his remarks in the name of Allah and asking for God's forgiveness.

“I pray to Allah to bestow his mercy upon the deceased,” he said. “I ask Allah to have mercy upon me, upon my brother, upon my family.”

Judge George O'Toole officially imposed the death sentence, which had been reached unanimously by the 12-person jury on May 15.

“I sentence you to the penalty of death by execution,” O'Toole told Tsarnaev, before he was led away by US Marshals.

Despicable

Throughout the trial the man who came to the United States as a child and took citizenship in 2012, sat in silence, at times fidgeting but expressing little emotion.

On Wednesday, more than 20 victims and their relatives addressed the court, making harrowing impact statements summing up their grief, pain, anger and at times forgiveness.

The first to speak was the mother of Krystle Campbell, a young woman killed in the April 15, 2013 bombings.

“The choices you made were despicable,” said Patricia Campbell, addressing Tsarnaev directly as he bowed his head.

“What you did to my daughter was disgusting. The jury did the right thing.”

Bill Richard, the father of the youngest victim, eight-year-old Martin, said he would have preferred Tsarnaev receive a life sentence but said the attacks were “all on him”.

Tsarnaev, Richard said, could have stopped his brother, changed his mind and “walked away with a minimal sense of humanity”.

“He chose hate. He chose destruction. He chose death,” Richard said.

“We choose love. We choose kindness. We choose peace. That is what makes us different.”

Defense lawyer Judy Clarke told the court that Tsarnaev had offered to plead guilty last year, but Wednesday's remarks were the first time that her client had expressed any public remorse.

To avenge Iraq, Afghanistan wars

At the penalty phase of the trial, a prominent Catholic nun, Sister Helen Prejean, who visited him in jail said that he did to her.

“No one deserves to suffer like they did,” she quoted him as saying.

The bombings wounded 264 people, including 17 who lost limbs, near the finish line at the northeastern city's popular marathon.

It took the jury more than 14 hours to choose death rather than life imprisonment for Tsarnaev on six counts.

It was a stinging defeat for the defence, who argued that Tsarnaev was a “lost kid” who would never have committed such horrors without being manipulated by his older brother.

The brothers went on the run and killed a police officer, before Tamerlan was shot dead and Tsarnaev arrested, four days later.

He was found, injured, in a grounded boat on which he had scrawled a bloody message defending the attacks as a means to avenge US wars in Iraq and Afghanistan.

The death sentence is possible only under federal law. The state of Massachusetts outlawed capital punishment in 1947 and opinion polls had suggested residents favored a life sentence for Tsarnaev.

During the trial, government prosecutors argued Tsarnaev was a remorseless terrorist who deserved to die and declared that life imprisonment would be the “minimum” punishment.