North and South Korea exchanged artillery fire in the Yellow Sea on Wednesday, a day after Pyongyang declared the border area a "no-sail" zone, the South Korean Yonhap news agency reported.
The two Koreas' disputed maritime boundary is often a source of tension on the peninsula, divided since the Korean War ended with a truce in 1953.
"We have confirmed North Korea's firing of several artillery shells, but they did not cross the [Northern Limit Line]," Park Sung-woo, of the Joint Chiefs of Staff in Seoul, was quoted by the news agency as saying.
"We are on high military alert," he said.
Media reports quoted an unidentified government official at South Korea's presidential office as saying South Korea had responded by firing warning shots.
"Our military fired at the shells in the air," he said. "Our field manual states that we are supposed to target any incoming flying objects."
The firing, from land-based artillery, reportedly caused neither casualties nor damage. No naval vessels were involved.
The North Korean People's Army General Staff confirmed the firing, saying it was part of an "annual" drill.
"No one can argue about the premeditated exercises staged by KPA units in waters of the north side," the General Staff said in a statement published by the North's Central News Agency.
"Such firing drill by the units of the KPA will go on in the same waters in the future, too," the statement said.
South Korea described the North's firing as a "grave provocation" that undermined the truce between the two sides and urged the communist neighbor to cancel its Tuesday's declaration of a "no-sail" zone around the inter-Korean sea border, Yonhap said.
The border was unilaterally drawn by the U.S.-led United Nations Command in the wake of the 1950-1953 Korean War and has been a sticking point between the North and the South. Pyongyang has not acknowledged the borderline and has drawn a new one on its own south of the current border. Naval clashes between the two states over the disputed area took place in 1999, 2002 and last year.
MOSCOW, January 27 (RIA Novosti)