The personal information of nearly 1,000 North Koreans who defected to South Korea has been leaked after unknown hackers got access to a resettlement agency's database, the South Korean Unification Ministry said on Friday.

The ministry said it discovered last week that the names, birth dates and addresses of 997 defectors had been stolen through a computer infected with malicious software at an agency called the Hana center, in the southern city of Gumi.

"The malware was planted through emails sent by an internal address," a ministry official told reporters on condition of anonymity, due to the sensitivity of the issue, referring to a Hana centre email account.

The Hana centre is among 25 institutes the ministry runs around the country to help some 32,000 defectors adjust to life in the richer, democratic South by providing jobs, medical and legal support.

Defectors, most of whom risked their lives to flee poverty and political oppression, are a source of shame for North Korea. Its state media often denounces them as "human scum" and accuses South Korean spies of kidnapping some of them.

The ministry official declined to say if North Korea was believed to have been behind the hack, or what the motive might have been, saying a police investigation was under way to determine who did it.