Surviving Boston Marathon bombing suspect Dzhokhar Tsarnaev has recovered enough to walk, according to his mother.

Zubeidat Tsarnaeva said her son told her in a phone call that he and his brother, who was killed in a police manhunt after the blasts, were innocent.

In a rare glimpse at Dzhokhar Tsarnaev's state of mind, he told her he was getting better and that he had a very good doctor, but was struggling to understand what happened, she said.

"He didn't hold back his emotions either, as if he were screaming to the whole world: What is this? What's happening?," she said.

Mr Tsarnaev, who was shot and injured during the manhunt, is currently being held in a prison hospital.

Last month's bombings left three people dead and more than 260 others injured.