A former spy chief convicted of intervening in the 2012 presidential election filed an appeal with the country's top court against the ruling on Thursday, his lawyer said.

Overturning a lower court ruling, the Seoul High Court sentenced Won Sei-hoon to three years in prison for willfully neglecting online posts made by his subordinates that slandered the opponents of President Park Geun-hye, a then presidential candidate, in the run-up to the election.

Won headed the state intelligence agency from 2008 to 2013. He stepped down after a public outcry over the allegations.

Won's attorney said the appeal was filed with the Supreme Court earlier on Thursday, saying the high court ruling was "nonsensical."

"The online posts were made to thwart North Korean attempts to skew South Koreans' opinions seen on the Internet," Won's attorney, Lee Dong-myung, told Yonhap News Agency by phone. "It was not the volition of the National Intelligence Service (NIS). We hope to delve into this matter further at the appellate trial."

Lee also said he is appealing the decision that Won was behind the online smear campaign.

"The NIS head couldn't possibly know the nitty-gritty of what goes on in the organization," the lawyer said. "Won simply ordered them to eliminate pro-North Korea forces."

Lee said he was hopeful about the outcome of the Supreme Court appeal.

"Because Won was acquitted of violating election laws at the first trial, I believe it is quite possible that we will win this one," he said. (Yonhap)