Gunmen opened fire on two Saudi police posts in the northwest of the Gulf kingdom killing two civilians and a policeman, the official news agency SPA reported on Friday.

It said two masked assailants shot dead two Saudis in Al-Shamli, in the Hail region, on Thursday and a gunman killed a policeman at the traffic police station in a second attack in the town half an hour later.

The news agency gave no motives for the attacks but said an investigation has been opened.

Security forces have been on the alert in the face of mounting attacks by the Islamic State jihadist group which controls swathes of Syria and Iraq.