SEOUL (AFP) - South Korean President Park Geun Hye vowed on Thursday to ensure the political neutrality of government agencies, as she addressed allegations that the domestic spy service interfered in last year's presidential election.

"Even if I myself did nothing that may arouse suspicion, suspicions have been raised that national agencies might have interfered with the election," Ms Park told a meeting of her top advisers.

"From now on, the government will make sure that not only government agencies ... but also individual public employees won't breach political neutrality even inadvertently," she said.

They were her first direct comments on a widening scandal that has included accusations of an attempted cover-up.

Opposition parties have spent weeks hammering Ms Park's administration over the poll-meddling issue, suggesting that it calls the legitimacy of her December election victory into question.

The allegations focus on the origin of multiple online messages, posted prior to polling day, that denounced opposition presidential candidate Moon Jae In as a North Korean sympathiser, while extolling the virtues of Ms Park, the ruling party candidate.

While initially dismissed as the work of a couple of maverick National Intelligence Service (NIS) agents, further investigations have suggested a wider, coordinated smear campaign.

The affair has already resulted in the arrest of former NIS chief Won Sei Hoon, who is now awaiting trial on multiple charges, including violating the national election law.

Ms Park said an ongoing federal investigation would end with the "truth revealed clearly before the people" and promised that anyone proved guilty of misconduct would be brought to account.