SEOUL (AFP) - Two members of South Korea's spy agency interfered in last year's presidential election by conducting an online smear campaign against the main opposition candidate, police alleged on Thursday.

Announcing the results of a four-month investigation, police said they had forwarded their findings to the state prosecutor to consider possible indictments.

The two agents, including a 28-year-old woman, used multiple IDs to post scores of messages and articles on popular websites that were highly critical of Democratic United Party (DUP) candidate, Mr Moon Jae-In, the probe said.

Mr Moon was defeated in the December 19 vote by the conservative candidate and current president, Ms Park Geun Hye.

The police investigation followed complaints from the DUP that the National Intelligence Service (NIS) was behind the campaign.

The NIS has firmly denied any institutional involvement.

"We will widen our investigation to see if other people were involved in the case," the police said in a statement.