South Korea has condemned North Korea's recent missile and artillery tests, including a live fire drill near the demilitarized zone separating the two countries.
Seoul defense officials say the North fired about 100 shells from multiple rocket launchers into the waters near the eastern sea border on Monday.
South Korean Defense Ministry spokesman Kim Min-seok on Tuesday said move was "threatening."
"North Korea fired from a location very close to the Demilitarized Zone. It represents such a threat to South Korea that even our civilian tourists were able to witness columns of water triggered by North Korean shells landing in the sea," said Kim.
Kim said that if any of the shells fall on the southern side of the border, South Korea will "retaliate without hesitation."
The shellings came a day after North Korea test-fired two ballistic missiles into waters separating Japan and the Korean peninsula.
North Korean state television on Tuesday released a series of undated pictures showing Kim Jong Un presiding over a rocket launch.
It was not immediately clear why North Korea conducted drills so close to the border, but state media has in recent days called the presence of a U.S. Navy aircraft carrier in South Korea a "sinister interference."
The United States, which has more than 28,000 troops stationed in South Korea, said it was concerned by reports of recent North Korean missile launches.
U.S. State Department spokeswoman Jen Psaki said it was "not appropriate'' to try to link the launches to joint military exercises between the United States and South Korea.
"These annual joint exercises are transparent, defense-oriented. They've been ... carried out regularly and openly for roughly 40 years now. And these recent missile launches were conducted without warning and are clearly designed to raise tensions,'' Psaki told a regular news briefing.
Pyongyang has conducted a series of rocket launches and artillery shelling in recent months, despite making several peace overtures to Seoul, including a decision to participate in the Asian Games to be held in late September and early October in the South Korean city of Incheon.
Representatives for both sides will meet Thursday at the border truce village of Panmunjom to discuss the North's participation in the Games.
Pyongyang plans to send 150 athletes and a squad of cheerleaders to the games.
Some information for this report provided by Reuters.