Seoul, South Korea (CNN) -- North Korea says artillery shells it fired Wednesday in waters near South Korea were part of an annual training drill.
The North Korea shelling led to the two countries exchanging fire Wednesday, South Korean state-run media reported.
Both countries fired in the air, without reports of casualties, the Yonhap News Agency quoted a South Korean official as saying.
"We have confirmed North Korea's firing of several artillery shells, but they did not cross" the two countries' maritime border, said Park Sung-woo, of Seoul's joint chiefs of staff, according to Yonhap. "We are on high military alert."
"Following the firing by North Korea, South Korea responded by shooting vulcan canons into the air, a statement that it would not be intimidated by saber-rattling by the communist neighbor," Yonhap said.
North Korea's state-run media said the shelling was part of its annual military training.
"Artillery units of the Korean People's Army staged an annual artillery live shell firing drill in waters of the West Sea of Korea Wednesday morning," the KCNA news agency said. "No one can argue about the premeditated exercises staged by KPA units in waters of the north side. Such firing drill by the units of the KPA will go on in the same waters in the future, too."
That exchange came a day after North Korea declared a "no sail zone" in a disputed area of the Yellow Sea, Yonhap said.
The two sides last exchanged gunfire in the disputed area in November, the first such violent incident in seven years.