North Korea denounced South Korea's spy agency on Sunday for what it said to be "unduly linking" the treason scandal surrounding a pro-North Korean lawmaker with the communist country, adding that it will not tolerate any attempts to hurt the on-going inter-Korean rapprochement mood.

The National Intelligence Service (NIS) formally arrested Lee Seok-ki, a legislator of South Korea's minor opposition Unified Progressive Party (UPP), and three other members of the so-called Revolutionary Organization (RO) on Thursday on charges of plotting to overthrow the government.

The spy agency accused them of conspiring to blow up key infrastructure and pledging to side with the North in case of a war on the Korean Peninsula.

Investigators also suggested that Lee and the members may have contacted Pyongyang as part of their conspiracy activities.

In a direct rejection of the accusation, North Korea's Rodong Sinmun said, "Their activities came out of their own will, not out of orders or directions from others.

"The NIS and the conservative forces are trying to make every scheme to falsely accuse UPP members of treason and to link it with us," the newspaper said in a news article. The mainstream daily is published by the North's governing Workers' Party of Korea and deemed to represent the official stance of the North Korean regime.

The Sunday article on the South Korean treason case marks the North's criticism over the scandal for the third day in a row.

The news article also said that the spy agency's attempt to embroil North Korea in the conspiracy case constitutes an "unpardonable insult" upon the North's efforts and seriousness toward improving relations with Seoul as well as a challenge to those who want the reunification of the two Koreas.

"It is a deliberately planned fabrication designed by those who don't want improvement in the inter-Korean relations and reduction of tensions," the Rodong Sinmun said, also adding that the country will not tolerate any attempt aiming to pose obstacles to improving inter-Korean relations.

Ending months of tension, the two Koreas dramatically reached an agreement last month to reopen the jointly run Kaesong Industrial Complex in the namesake North Korean border city.

The countries are also in discussion to hold reunion events to bring together family members separated by the 1950-53 Korean War. (Yonhap new)