SMS notifications have been sent to more than 700,000 patients to inform them if their data were stolen in the cyberattack, the health operator said in a statement.

These individuals had visited SingHealth's specialist outpatient clinics and polyclinics between May 1, 2015 and July 4 this year, which is the period when 1.5 million patient records were accessed and copied by hackers.

The remaining SMS notifications will be sent over the next two days, SingHealth said.

About 150,000 patients do not have their mobile phone numbers registered with SingHealth; these individuals will receive letters within one week to inform them if they are affected.

SingHealth's update on patient engagement came a day after news broke that its IT system had been a target of the "most serious" breach of personal data in Singapore's history.

The cyberattack, which occurred from June 27 to July 4 this year, led to 1.5 million SingHealth patients' records accessed and copied, of which 160,000 had their outpatient dispensed medicines' records taken.

The information copied during the hack includes NRIC numbers, addresses, gender, race and date of birth, SingHealth said.

No phone numbers, financial information or other patient medical records were illegally accessed.

"We apologise unreservedly to patients for the anxiety caused and will continue to do all that we can to reach out to them," SingHealth said in its update.

SingHealth also said that about 139,000 patients have accessed the Health Buddy mobile app and its website to check if they are affected.

The health operator has also received more than 4,800 calls from patients and members of the public enquiring about the cyberattack, and close to 750 emails.