Two Saudi border guards have been killed and a third wounded in a suicide bomb attack on the kingdom's frontier with Iraq, the interior ministry says.
Two armed men opened fire on the guards at 04:30 (01:30 GMT), near Judayyidat Arar in the Northern Borders province.
One of the assailants was shot dead by the guards, but the other detonated an explosive vest he was wearing.
No group has said it was behind the attack, but Saudi Arabia has joined the US-led coalition against Islamic State.
Royal Saudi Air Force jets are conducting air strikes on the jihadist group's positions in Syria, along with three other Arab states.
The interior ministry said the security services had launched an investigation into Monday's attack.
One of the guards who was killed was a senior officer, an interior ministry spokesman told Reuters news agency. The officer was named by local media as Gen Oudah al-Belawi.
Saudi Arabia's heavily-defended border with Iraq has been come under mortar fire on previous occasions, but more direct assaults are rare.
The kingdom deployed thousands of extra troops along the frontier in July after IS seized large swathes of territory in Iraq and Syria.
Judayyidat Arar is close to the border with Iraq's western province of Anbar, parts of which are controlled by IS and its Sunni Arab allies.