Singapore's Prime Minister Lee Hsien Loong, left, talks with US Defense Secretary Jim Mattis. Picture: Yong Teck Lim/AP

Singapore - Patient data of 1.5 million people in Singapore, including the country's Prime Minister Lee Hsien Loong, have been stolen in a breach that the government has termed "serious, unprecedented [and] massive."

Investigations by the Cyber Security Agency (CSA) found that victims' name, national identification number, address, gender, race and date of birth had been stolen between May 1, 2015, and July 4, 2018, the health and communications ministries said Friday. 

In addition, information on medicines that had been dispensed to 160,000 of the 1.5 million affected patients had also been stolen, including information about medication prescribed to the prime minister.

"The attackers specifically and repeatedly targeted Prime Minister Lee Hsien Loong's personal particulars and information on his outpatient dispensed medicines," the statement said.

However, the evidence did not suggest that any records had been amended or deleted, and no other patient records, such as diagnosis, test results or doctors' notes, had been breached.

A Committee of Inquiry will be established and immediate action will be taken to strengthen government systems against cyber attacks, the Ministry of Communications said in a separate statement.

Lee said in a Facebook post following the announcement that he was not sure what the attackers were hoping to find, saying that there was "nothing alarming" in his medical records.

The latest major cybersecurity attack on the city-state was in 2017 when a breach at the defence ministry resulted in the theft of personal data of 850 national servicemen and employees.