South Korea is refusing to delay joint military drills with the United States, as requested by North Korea during high-level talks this week.
The annual war games are set to begin later this month, around the same time as planned reunions between families separated by the Korean War.
Defense Ministry spokesman Kim Min-seok said the North's threat to call off the reunions because of the exercises was inappropriate.
"It is wrong to relate the defensive nature of military drills, which are for national security, to the separated family reunions, which has a humanitarian objective," said Kim.
The two sides on Wednesday held their highest-level talks in seven years. Seoul said Pyongyang demanded a two-day delay in the drills.
The North has for weeks asked for a cancellation or, more recently, a relocation of the drills. It views them as an unacceptable rehearsal for invasion.
The U.S. and South Korea insist the drills were planned long in advance and are defensive.
The reunions, if they take place, will happen from February 20-25 at the North Korean resort area of Mount Kumgang.
The reunions have not been held since 2010. In September, North Korea cancelled the reunions at the last minute, citing South Korean hostility.
Pyongyang has recently given signs it is open to improved relations with Seoul. But it has also attempted to portray the war drills as evidence Seoul is not interested in peace.
Tensions regularly flare up between the two Koreas, which remain in a technical state of war, since the 1953 agreement that ended hostilities between them was only a truce.