The Saudi royal family are building a 600-mile barrier to fortify the northern frontier of their kingdom.
The fence and ditch, punctuated with radar surveillance towers, command centres and guard posts, aims to protect the Saudis’ oil-rich territory from invasion by the Islamic State insurgency.
Last week a suicide bombing and gun attack which killed two Saudi border guards and their commanding officer was styled by one analyst as the Islamic State’s first attack on the kingdom.
No group claimed responsibility for the assault in a remote desert area, but it happened just next to Iraq’s Anbar province where Islamic State militants are fighting Iraqi army forces.
Saudi Arabia’s King Abdullah bin Abdulaziz al-Saud inaugurated the first phase of the border security project in September, soon after Islamic State’s Sunni insurgency swept across Iraq.