A former Seoul police chief accused of meddling in last year's presidential election refused to take an oath at a parliamentary hearing Friday, citing an ongoing trial on the case.

Kim Yong-pan is one of two key witnesses in a parliamentary probe into allegations that the National Intelligence Service (NIS) conducted an online smear campaign to sway public opinion in favor of the ruling Saenuri Party ahead of December's presidential election.

Kim, the then Seoul police chief, has been accused of reducing the scope of a police investigation into the scandal and whitewashing its results.

The other key witness is former NIS chief Won Sei-hoon, who allegedly ordered the online smear campaign. Both were indicted in June on charges of interfering in the election.

"Due to this incident, a criminal trial is going on at the same time as the parliamentary investigation," Kim said at the beginning of the hearing after refusing to take the oath.

"If the truth of my testimony is distorted or misinterpreted in the process of being disclosed through the press, this could affect (my) trial."

Under the country's law, witnesses standing trial can refuse to give testimony in parliament if they believe that will have a negative influence on their trials.

Still, Kim began to answer lawmakers' questions selectively, denying all the allegations against him.

Meanwhile, Won is expected to appear before the parliamentary investigative committee later in the day, said Rep. Yoon Sang-hyun, the first vice floor leader of the ruling party, in a phone call with Yonhap News Agency. (Yonhap News)