Accused bomber Dzhokhar Tsarnaev should be put to death if he is found guilty of planting devices that killed three people and wounded 264 at the Boston Marathon last year, the US government's chief prosecutor has said.

Attorney General Eric Holder said in a statement on Thursday that he was authorising trial prosecutors to seek the death penalty against Tsarnaev, who is charged with committing one of the worst attacks on US soil since the September 11, 2001, attacks.

"The nature of the conduct at issue and the resultant harm compel this decision," Holder said.

Holder had faced a Friday deadline for deciding whether to seek the death penalty as part of Tsarnaev's upcoming trial.

Prosecutors say that Tsarnaev, 20, and his 26-year-old brother Tamerlan, planted a pair of homemade pressure-cooker bombs at the race's crowded finish line on April 15, 2013, killing three people and wounding 264.

Three nights later, the pair killed a university police officer and later engaged in a shootout with police that left Tamerlan dead, prosecutors say.

The younger Tsarnaev has pleaded not guilty to criminal charges.

US justice department officials said the nearly seven months since the attacks was necessary to fully evaluate the circumstances of the case and to gather recommendations from prosecutors advising Holder.

Holder has said he is not a proponent of the death penalty because he believes its value as a deterrent is questionable.

Since becoming attorney general in 2009, he has authorised prosecutors to seek the death penalty in 36 cases, according to the justice department.

Source: Reuters