A statement by South Korea's Joint Chiefs of Staff (JCS) confirmed Monday that "North Korea fired two short-range missiles with a range of some 490 kilometers into the East Sea from its western port city of Nampo between 6:32 a.m. and 6:41 a.m."
Meanwhile, joint forces from the U.S. and South Korea began Key Resolve and Foal Eagle -- a pair of exercises that will mobilize hundreds of thousands of troops for several weeks.
The U.S. has nearly 30,000 military personnel stationed in the South.
While the allies insist the drills are defensive, North Korea has regularly condemned them as preparations for war.
A spokesperson for the General Staff of Pyongyang's Korean People's Army was quoted by state-run KCNA news agency as warning that "the U.S. and South Korea will have to bitterly regret the irretrievable consequences to be entailed by Key Resolve and Foal Eagle."
"They should be dealt with only by merciless strikes," the statement cautioned.
North Korea had already carried out two other missile launches this year, but South Korea's JCS said the latest show of force "appears to be the North's provocations in opposition to the Key Resolve and Foal Eagle exercises."
Seoul's defense ministry also threatened the South would "respond in a stern and strong manner based upon a staunch combined defense posture."
"Its menacing words and deeds are a very grave challenge to the security of the Korean Peninsula and firing ballistic missiles is in violation of the United Nations' resolutions," ministry spokesperson Kim Min-seok told reporters.
The Koreas technically remain at war after an armistice, rather than a peace treaty, brought the 1950-53 Korean War to a close.