A Saudi soldier has been killed during a raid against militants in the town of Al Awamiya in the eastern Qatif province, Al Arabiya TV reported.
Violence in the area had appeared to abate since security forces flushed out most of the gunmen last summer.
Al Arabiya identified the officer on Saturday as Ahmed Hassan Al Zahrani and said he was killed "by extremists", without providing details.
"During the raid on Thursday evening, several weapons and explosives were found. Security forces foiled the terrorist plots after the operation," Al Arabiya said.
Discontent among Qatif's Shiite community and general crime have driven a wave of violence in the area, along with ISIL-claimed attacks.
Since 2011, Saudi security forces have occasionally been attacked in Al Awamiya, where some residents have accused the government of marginalisation — something Riyadh denies.
Authorities have been bulldozing Al Masoura, a historic neighbourhood with narrow streets dating back more than 200 years, to make way for an urban renewal project. The demolitions prompted armed clashes between militants and security forces.
Local officials say wanted “terrorists” believed to be behind attacks on security forces have been using the narrow streets and alleys to escape arrest.
Saudi authorities said a Shiite judge was abducted and killed by wanted militants in December, and a militant suspect was shot and killed in January in a shoot-out.