Kim Yo-jong, who is a senior official in the North's ruling Workers' Party, also said a change made to the plan for annual joint military drills by South Korea and the United States was a "futile" move that does not change the allies' hostile intent.
Kim Yo-jong, who officials and analysts believe speaks for her brother, has in recent weeks rebuffed moves taken by South Korea's new liberal government aimed at easing tension between the two Koreas.
"I am confident that Seoul's policy towards the DPRK remains unchanged and can never change," she was quoted as saying by KCNA official news agency. DPRK is short for the Democratic People's Republic of Korea, North Korea's official name.
South Korea's military has said it detected moves by the North's military to dismantle some propaganda loudspeakers directed at the South, following similar moves by the South.
Seoul's Joint Chiefs of Staff on Thursday said it stood by its assessment of activities it had observed at some parts of the border, adding it was continuing to monitor the situation.
The military's spokesman Lee Sung-jun said he believed caution was needed when interpreting statements made by the North to avoid being misled and that Pyongyang had often made "claims that are untrue." He declined to elaborate.
Lee did not directly address a question about a news report that the North had taken down only one loudspeaker out of the dozens it had positioned along the border.
There has been cautious optimism in the South that the North may be responding positively to a policy by President Lee Jae-myung to engage Pyongyang after a period of cross-border tension and even show willingness to return to dialogue.
Kim Yo-jong also said North Korea would not be sitting down with the United States for dialogue, saying reports raising the possibilities of such a development were "false suppositions".
