North Korea is demanding South Korea and the United States cancel upcoming annual military drills, saying that the drills, if held, will "fatally destroy" inter-Korean relations.
Pyongyang's state media warned of "unimaginable calamities and disasters" if the Foal Eagle and Key Resolve exercises proceed as scheduled late next month.
South Korea's Defense Ministry dismissed the threats Thursday. Spokesman Kim Min-seok warned Seoul will "mercilessly and decisively punish" Pyongyang in the event of a provocation.
"We will conduct Key Resolve and Foal Eagle Exercise as planned. If North Korea actually commits military aggression at the excuse of what is a normal exercise we conduct as preparation for emergencies, our military will mercilessly and decisively punish them," said Kim.
The annual military drills are a routine source of tension between the two longtime foes. The North views them as preparation to invade, while the South maintains they are defensive in nature.
Last year's drills came at a particularly tense period of inter-Korean relations, after Pyongyang carried out its third nuclear test. At its worst point, the North was issuing near daily threats of nuclear war against the U.S.-backed South.
Inter-Korean relations had shown signs of improvement in recent weeks, with both sides at least speaking of the need to improve ties. Both sides have also accused the other of ratcheting up tensions.
South Korea is watching the North closely following last month's execution of Jang Song Thaek, the powerful uncle and mentor of North Korean leader Kim Jong Un. Jang was accused of plotting a coup, raising fears of instability in Pyongyang.
Some analysts have said North Korean leaders may carry out another nuclear or long-range missile test in order to build internal unity. Seoul's Defense Ministry said Thursday it is closely monitoring Pyongyang's military movements, but said there has been "nothing out of the ordinary."
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.