Both South and North Korea have held a series of live-five artillery drills as tensions between the two nations mount over Pyongyang's threat to end non-aggression pacts with Seoul.
South Korean and U.S. military forces held a series of joint exercises Thursday as part of an ongoing training operation.
Meanwhile, South Korean Prime Minister Jeong Hong-Won said his country will respond sternly to any attacks from the North.
Prime Minister Jeong spoke Thursday while visiting the western border island of Yeonpyeong to inspect its preparedness to respond to any type of provocation from the North.
The island was the site of a 2010 North Korean artillery attack that left two civilians dead.
Also Thursday, North Korean state media report that leader Kim Jong Un recently watched a live-fire drill by artillery units on two islands in the Yellow Sea. The reports did not specify when the drill took place.
North Korea has threatened a nuclear war against the United States and made a series of threats against the South after the United Nations last week imposed sanctions for its latest nuclear test.
It is not believed to have the capacity to attack the U.S. with a nuclear weapon. But there is fear it may attack areas near the disputed border, which has been the site of several deadly clashes in recent years.
Pyongyang already claims to have scrapped an armistice agreement that ended the Korean war in 1953, although U.N. and South Korean officials say this cannot be done unilaterally.
U.S. President Barack Obama on Wednesday urged North Korea to take "confidence-building measures" aimed at diffusing tensions, such as ending its nuclear and missile testing.
In an interview Wednesday with the U.S. television network ABC, Mr. Obama said countries will reciprocate if they see any kind of responsible behavior from North Korea. He said Pyongyang has yet to display that type of behavior.