Saudi Arabia is executing people at a "frightening" rate, campaigners have warned, after it emerged the kingdom had killed its 70th prisoner of the year bringing the total number to almost half that of 2015.

Alaa al-Zahrani, who was put to death in the Red Sea city of Jeddah, was found guilty of killing fellow Saudi Abdullah al-Sumairi with a rock to the head, the interior ministry revealed in a statement published by the Saudi's official state news agency SPA. 

Several advocacy groups that monitor the death penalty worldwide placed the figure at 157 last year, with beheadings reaching their highest level in two decades. 

Saudi Arabia does not release annual tallies, though it does announce individual executions in state media throughout the year. Most people beheaded in the oil-rich kingdom are beheaded by sword.