North Korea fired about 110 rounds Monday near its disputed western sea border with South Korea, shortly after Seoul ended five-day naval drills staged in response to the March sinking of its Cheonan warship. North Korea has been blamed for the vessel's destruction, which killed 46 sailors.
The North denies it torpedoed the Cheonan and has warned any punishment would trigger war.
Most of the shells in Monday's barrage landed in the North's waters, but about 10 struck near a South Korean island near the sea border causing no damage, the Joint Chiefs of Staff said. It previously stated all artillery rounds landed in North Korean waters.
South Korea's Defense Ministry said in a statement that the move violated an armistice that halted fighting in the Korean War in the early 1950s.
"If North Korea continues its provocative rhetoric and acts, we will sternly deal with them," the ministry said.
The North, meanwhile, threatened a "war of retaliation."
North Korea "will clearly show to those buoyed by war fever what a real war is like any time it deems necessary through a war of retaliation of its own style based on its nuclear deterrent," the main Rodong Sinmun newspaper said in a commentary carried by the official Korean Central News Agency.
The Korean peninsula remains technically in a state of war because the 1950-53 Korean War ended in a truce, not a peace treaty. The U.S. stations 28,500 troops in South Korea to help protect its key Asian ally.
A U.S. State Department spokesman denounced North Korea's artillery barrage and joked that it "resulted in a lot of dead fish." P.J. Crowley also said, "We're likely to see more provocations."
He said it was unclear what North Korea was "trying to achieve through this ongoing chest-thumping."
Tension on the divided peninsula has spiked in the wake of the Cheonan's sinking.
The American-led U.N. Command, which monitors the armistice, held talks with North Korea at the Korean border village of Panmunjom on Tuesday to discuss the incident, said U.S. military spokesman David Oten.
It was the fourth such meeting since last month. The previous three ended with no major breakthroughs.
Ties between the two Koreas worsened last week when South Korea launched large-scale naval training in response to the sinking, prompting its communist neighbor to warn it would counter the maneuvers with a "strong physical retaliation."
The South Korean naval drills, which ended on Monday afternoon, followed large-scale joint military exercises with the U.S. last month off the east coast. The South Korean drills included exercises near the western sea border where the warship exploded and sank.
The area is also where the navies of the rival Koreas have fought three bloody gunbattles since 1999.