Rival political parties on Wednesday agreed on a list of witnesses they will summon for a parliamentary probe into the state spy agency's alleged meddling in last year's presidential election.

The agreement comes after weeks of wrangling between rival parties over who should stand as witnesses and how to ensure their appearance at the hearings.

The probe, which began early last month, aims to uncover the truth behind 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.

The main opposition Democratic Party (DP) has demanded that then NIS chief Won Sei-hoon and then Seoul police chief Kim Yong-pan appear before the parliamentary investigative committee for questioning in connection with the scandal.

Won has been accused of ordering the alleged online campaign, while Kim is suspected of reducing the scope of a police investigation into the scandal.

The DP has also insisted that the investigation committee issue an order compelling the two to comply with the summons if they fail to show up.

On Wednesday, the rival parties jointly announced a list of 29 witnesses they plan to summon for hearings later this month.

The parties earlier agreed to extend the probe by eight days until Aug. 23 and postpone the hearings to take place over the three days of Aug. 14, 19 and 21.

The list includes Won and Kim alongside other former and current NIS and police officials, Rep. Kwon Seong-dong of the ruling party and Rep. Jung Chung-rai of the opposition party said in a joint press conference at the National Assembly.

Won and Kim will be summoned on Aug. 14, while the others will be summoned on Aug. 19, they said.

The parties plan to continue negotiations over other witnesses they would like to summon and request their appearance at the final hearing on Aug. 21, they added.

The lawmakers did not specify who the other potential witnesses are, but the opposition party has demanded the appearance of Rep.

Kim Moo-sung of the ruling party and Ambassador to China Kwon Young-se.

The two have been accused of obtaining a classified transcript of the 2007 inter-Korean summit from the NIS ahead of last year's presidential election to use it as part of a smear campaign against then opposition presidential candidate Moon Jae-in, a former chief of staff to late President Roh Moo-hyun.

The ruling party has claimed that the transcript, which the NIS recently declassified and disclosed to the public, shows that Roh undermined the country's sovereignty during his summit with then North Korean leader Kim Jong-il by allegedly offering to surrender the inter-Korean Yellow Sea border.

The opposition party has rejected the claims.

Despite Wednesday's agreement, some expressed doubts that Won and Kim will testify even if they appear at the hearings.

"Even if a compliance order is issued and they are brought to the hearings, the reality is that there is no way to force them to testify if they refuse to do so," Rep. Kim Jae-won, who represents the ruling party on the investigative committee, told Yonhap News Agency by phone.

In June, prosecutors indicted Won and Kim on charges of meddling in the presidential election, and under South Korean law, defendants may refuse to testify before the National Assembly if they believe that will influence their trials. (Yonhap News)