The prosecution said Friday it has found evidence that agents of the state spy agency quoted press releases of the ruling party to post Twitter messages ahead of last year's presidential election.

The National Intelligence Service (NIS) has been probed over allegations that it launched an online smear campaign to sway public opinion in favor of President Park Geun-hye, the then ruling Saenuri Party's candidate, ahead of the December election.

The episode has already resulted in the arrest of former NIS chief Won Sei-hoon and had appeared to have come to a close but regained traction after fresh findings suggested the alleged operation was much more extensive than originally thought.

The new discoveries by an investigation team at the Seoul Central District Prosecutors' Office showed that the alleged operation was not limited to comments on major Internet bulletin boards but extended to Twitter, with NIS agents allegedly posting and retweeting nearly 55,000 messages.

Of those Twitter messages, nearly 50 of them were allegedly in favor of the Saenuri Party or the contents were similar to the party's press release, state prosecutors said, quoting Won's arraignment filed with a Seoul court.

Some of the Twitter messages allegedly were linked to the Saenuri Party's press releases that propagandized Park's election camp activities, in violation of the country's election law, prosecutors added.

The prosecution team probing the case already asked the Seoul Central District Court to file additional charges against Won. The court will make the decision next Wednesday whether to accept the prosecution's request.

The NIS, meanwhile, refuted the claim, saying that such Twitter messages were not tweeted or written by its agents.

"It has been confirmed that all of the retweeted messages were not from Twitter accounts belonging to NIS agents," the agency said in a press release, adding that it will file a lawsuit against a local daily to demand a correction for raising such allegations in the first place. (Yonhap News)