A suicide bomber killed two Saudi guards on Monday on the border with Iraq, where Islamic State jihadists have seized a swathe of territory, the interior ministry said.
The blast in the Arar region followed a firefight between the border patrol and the assailants, one of whom was shot dead, a ministry spokesman said in a statement carried by the official SPA news agency.
"A border patrol in Suwayf, in the northern Arar region, came under fire by terrorist elements," the spokesman said.
The Arar border post is the main crossing between Saudi Arabia and Iraq, who share a 800 kilometre (500 mile) border ©Ahmad al-Rubaye (AFP)
As forces killed one of the assailants, another attacker "detonated an explosive belt he was carrying," killing himself and two guards and wounding another.
The ministry did not specify how many assailants in total were involved in the dawn attack or their motives. It said an investigation was under way.
In July, three shells fired from inside Iraq hit the Arar area, without causing any casualties.
In November, Iraqi Shiite group Jaish al-Mukhtar claimed it had fired six mortar rounds into a remote area of northeastern Saudi Arabia as a "warning" to the kingdom.
Saudi Arabia shares a more than 800 kilometre (500 mile) border with Iraq.
The kingdom has been taking part in US-led air strikes against IS in Syria, in a move that has drawn threats of retaliation from the jihadists.
In a purported audio recording released on social media networks last month, IS leader Abu Bakr al-Baghdadi warned Saudi leaders they would see "no more security or rest."
Last month, Saudi Arabia said it had arrested 135 suspects for "terrorism" offences.
The authorities said they had arrested three IS supporters for shooting and wounding a Dane in November.
A week after that attack, a Canadian was wounded in a stabbing while he shopped at a mall in Dhahran on Saudi Arabia's Gulf coast. Police arrested a Saudi suspect.
In November, Saudi Arabia blamed IS-linked suspects for the killing of seven Shiites, including children, in the kingdom's oil-rich Eastern Province.
In October, a Saudi-American former employee of a US defence contractor shot dead an American colleague and wounded another in Riyadh.