Mohammed al-Fahimi, a spokesman for northern region border guards, said "the depth of the border has been increased by 20 kilometres (12 miles)", the Saudi Press Agency reported.
Officers guarding the frontier "called on residents and citizens to stay away from the border areas", it added, without clarifying the previous depth of the border zone.
In early September, the kingdom inaugurated a multilayered fence, backed by radar and other surveillance tools, along its northern borders.
The project is part of efforts to secure the kingdom's desert frontiers against infiltrators and smugglers, state media reported at the time.
Saudi Arabia shares a boundary of 800 kilometres with Iraq.
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In July 2009, Riyadh signed a deal with European aerospace and defence contractors EADS to build a high-tech security fence along thousands of kilometres of the kingdom's borders, not only in the north.
Since September, Saudi Arabia has been part of the US-led coalition bombing Islamic State group extremists in Syria.
Saudi Arabia has not, however, participated in strikes on IS in Iraq where the Sunni extremists have also seized territory.
Iraqi President Fuad Masum visited Saudi Arabia last week in a sign of warming relations after years of strain with the Sunni-dominated kingdom.
In November last year, an Iraqi Shiite group claimed it had fired six mortar rounds that hit a remote area of northeastern Saudi Arabia.
Saudi border guards said the rounds landed in Hafr al-Batin near Kuwait, but caused no damage.