South Korea's former spy chief has been indicted on charges of meddling in last year's presidential elections by ordering an online smear campaign against opposition candidates.

Won Sei-hoon ordered his agents to post comments slandering liberal candidates and praising the conservative Park Geun-hye, the Seoul central district prosecutors' office said. Park won the election in December by a million votes and took office in February.

Won resigned as head of the National Intelligence Service in March after serving four years under the conservative president, Lee Myung-bak.

Prosecutors said NIS agents tried to portray the opposition candidate Moon Jae-in as being too soft on North Korea and unqualified to protect South Korea against North Korean threats. NIS said it planned to issue a response later.

Under agency regulations Won would face up to five years' imprisonment if found guilty. This week the opposition party threatened to impeach the justice minister over the scandal. It is not clear if or how the effort affected the elections.