A Saudi policeman was killed and two others were wounded in an attack in a Shia-dominated area of Eastern Province, the interior ministry said Wednesday.
"One policeman was killed and two injured," an interior ministry spokesman said.
In a statement, police said two suspects were arrested after the patrol came under fire in al-Jesh village in Qatif district late Tuesday.
It was not immediately clear whether the incident was linked to unrest among the minority Shia community or to the Islamic State group of jihadists.
Residents of the region also complain of general criminal activity.
Most of Saudi Arabia's Shias live in the oil-rich east, where many say they face marginalisation.
Eastern Province was the scene of periodic clashes involving security forces after demonstrations broke out four years ago alongside a Shia-led protest movement in neighbouring Bahrain.
Since late last year, the region has been targeted by the Islamic State, a Sunni extremist group which consider Shias to be heretics.
Authorities, and analysts, say the jihadists have tried to ignite sectarian tensions in the Sunni-majority kingdom but have failed.
On successive Fridays in May suicide bombings at Shia mosques in Eastern Province, one of them in Qatif, killed a total of 25 people.
An IS-affiliated group calling itself Najd Province -- which takes its name from the region around Riyadh -- claimed those attacks as well as another suicide bombing that killed 26 people at a Shiite mosque in Kuwait last month.
Saudi security forces have also been targets of attacks linked to IS, which has committed atrocities in Iraq and Syria and inspired attacks elsewhere around the world.
On July 18 the interior ministry announced it had disrupted a network linked to IS and made more than 430 arrests, foiling new attacks on Shia mosques and a diplomatic mission.