The 19-year-old college student wanted over the Boston Marathon bombings is seriously ill in hospital and unable to be questioned after a manhunt that left the city virtually paralysed and his older brother and accomplice dead.

Boston Marathon bombing suspect Dzhokhar Tsarnaev has been captured alive and is in police custody, it has been confirmed.

The suspect being hunted in the Boston Marathon bombing is in a boat stored in a Watertown neighbourhood, a law enforcement official says.

US President Barack Obama called Boston mayor Tom Menino and Massachusetts governor Deval Patrick to offer federal assistance in responding to explosions that rocked the end of the Boston Marathon.