Saudi interior ministry spokesmen could not immediately be reached by telephone for comment on the reports by community and political activists on social media about the violence in Awamiya.
The area, part of the oil-producing Qatif province where some of the country's Shi'ite Muslim minority live, has seen unrest and occasional armed attacks by suspected militants on security forces since the 2011 Arab Spring protests.
Residents of the town complain of marginalisation by the Sunni-led Saudi government, a charge Riyadh denies.
Activists who write on community websites and use social media said three local residents and two expatriate workers have died since the security operation began early on Wednesday when a column of armoured vehicles entered Awamiya.
Since May, workers have also been tearing down structures in the 200-year-old walled old town district of al-Musawara, in what officials say is a plan to deny militants places to hide.
Several policemen have been killed in attacks by militants since that demolition work began.
Since the security forces advanced into the town, residents have reported the sounds of extensive shooting and explosions.
It was not immediately clear if armed men wanted by the authorities had fired back at the security forces.
Activists posted photos of several people they said had been hit, including the dead. Others have been wounded, but no precise figures were immediately available.
They identified the three local residents killed as Mohsen al-Awjami, Mohammed al-Faraj and Hussein Abu Abdullah, but gave no details on the expatriates.
A relative of Awjami said the 42-year-old man was returning home from work when he was shot by security forces.
"When he got out of the car, he was hit by a salvo of gunfire from security forces. He was struck in the abdomen."a family member told Reuters.
"He was outside for 2-1/2 hours before people managed to pull him inside and for an ambulance to take to hospital. He is now at the morgue at Qatif hospital."
Officials from the region's main Qatif Central Hospital declined to comment when reached by telephone, saying they were not authorised to speak to the media.
Activists posted a picture of another dead man they identified as Faraj, a bearded young man on a dark road with a bulge in his side, apparently from a projectile that hit him.
The authenticity of the posting could not be verified.
Authorities accuse militants in the area of a wave of attacks on security forces and a campaign of intimidation against local Shi'ites whom they accuse of cooperating with Saudi authorities.
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