The firing came shortly after South Korea ended five-day naval drills off the west coast that the North called a rehearsal for an invasion, vowing to retaliate.
All the artillery shells harmlessly landed into the North's waters and caused no damage to the South, a South Korean Joint Chief of Staff officer said on condition of anonymity, citing department policy.
South Korea considered the firing to be part of a military drill by North Korea but still bolstered its military readiness against further provocation, the officer said. The South also warned Pyongyang over the firing by naval radio, he said.
"This was their way of saying 'We'll respond to military drills with military drills,"' said Yang Moo-jin, a professor at Seoul's University of North Korean studies.
Yang said the firing is also aimed at highlighting the instability of the Korean peninsula to apply pressure on the United States to start talks on the signing of a peace treaty to formally end the 1950-53 Korean War. The conflict ended with an armistice, thus leaving the peninsula technically at state of war.
North Korea has long sought a peace treaty and diplomatic relations with Washington to guarantee that the U.S., which has 28,500 troops in South Korea, would not invade and topple Kim Jong Il's government. The U.S. has repeatedly said it has no intention of attacking the North.
Tension on the Korean peninsula is running high in the wake of the March sinking of the South Korean warship that an international investigation blamed on a North Korean torpedo attack. The North flatly denies the accusation and has warned any punishment would trigger war.
Tension deepened 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."
On Sunday, North Korean authorities also seized four South Korean and three Chinese fishermen aboard a 41-ton South Korean fishing boat for an alleged violation of the North's exclusive economic zone. South Korea has demanded the North quickly release them but the North has not responded.
China - the North's main benefactor and traditional ally - expressed concern over news of the seizure but said its diplomats in North Korea are verifying the report with local authorities, the state-run Xinhua News Agency reported Monday night.
Once the incident is confirmed, the North Koreans should treat the Chinese crew members well, guarantee their rights and inform Chinese officials, Xinhua reported, citing consulate officials in Chongjin, North Korea.
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 Cheonan exploded and sank, killing 46 sailors.
The area is also where the navies of the rival Koreas have fought three bloody gunbattles since 1999.
Associated Press writers Sangwon Yoon in Seoul and Cara Anna in Beijing contributed to this report.