SEOUL (AFP) - South Korea's former spy chief was jailed for three years Monday after an appeals court ruled that he had intentionally sought to influence the result of the 2012 presidential election.

Won Sei Hoon, 64, had been convicted in September last year of illegally engaging in political acts as head of the National Intelligence Service (NIS).

The charges related to an online smear campaign by NIS agents against the opposition party candidate whom the current president, Park Geun Hye, defeated in the 2012 poll by a narrow margin.

A district court gave Won a two-and-a-half year suspended sentence, ruling that while there was clear evidence of wrongdoing, there was not enough proof that he had directly sought to influence the outcome of the ballot.

The Seoul High Court, however, dismissed the lower court's distinction and deemed the crime serious enough to warrant a custodial sentence.

"It is fair to say Won had the intention to intervene in the election," Judge Kim Sang Hwan was quoted as saying by the Yonhap news agency.

Before he was taken from the court, Won insisted he had only ever worked "for the safety of my country and its people".

The spy agency, which has changed titles over the years, had a particularly notorious reputation in the decades of authoritarian rule before South Korea embraced democracy in the 1980s.

The modern-day NIS has been tainted by a series of scandals, most recently the forging of documents to build a false spying case against a former Seoul city official who had escaped to South Korea from the North in 2004.

Won's successor, Nam Jae Joon, publicly apologised over the forgery case in April last year and vowed a "bone-crushing" overhaul of the embattled agency.

A month later Nam was gone, to be replaced by the current NIS director Lee Byung Kee who has promised to distance the agency from domestic politics.