An international rights group says Egypt executed three Muslim Brotherhood members who were sentenced to death following torture and beatings to extract confessions.

Friday's statement from Human Rights Watch says families of the men were notified of their hanging on Thursday and asked to come to the morgue to pick up the bodies.

The three were convicted last year for the 2014 killing of a judge's son in the Nile Delta town of Mansoura.

HRW says a letter from one of the men was reportedly smuggled out of prison, describing torture and beatings.

Michael Page, deputy director at the New York-based group, says Egypt "committed a glaring injustice by executing three men who apparently gave 'confessions' extracted through electric shock and other torture."

There was no immediate comment from authorities.