North Korea fired about 500 artillery shells into South Korean waters on Monday – which the U.S. called "dangerous and provocative" – and the South responded by firing back. The tension on the border forced evacuations in five front-line South Korean islands.
The two rival nations traded fire into the Yellow Sea, north of the Koreas' disputed maritime boundary, according to The Associated Press.
The North fired 500 rounds of artillery shells for over three hours, and about 100 of them fell south of the sea boundary, according to South Korean Defense Ministry spokesman Kim Min-seok. The South also fired about 300 shells into North Korean waters.
The residents of South Korea's Baengnyeong island were evacuated to bomb shelters as a precaution. No injuries have been reported.
While it is routine for North Korea to test-fire artillery and missiles into the ocean, Pyongyang this time announced its plan in advance.
The North began its live-fire drills a little over noon Monday, about four hours after notifying the South through a fax message that it had zoned off seven areas just north of the Northern Limit Line for the drills, The Korea Herald reported.
Pyongyang had signaled it would conduct the exercise in response to U.N. condemnation of its missile launches last week and to oppose military drills by U.S. forces in the South, according to Reuters.
The North had also alleged that the South had "abducted" one of its fishing boats. The South, however, maintained that the boat had drifted into its waters and had been sent back.
Amid the drills, the U.N. Command Military Armistice Commission sent a message to the North, urging an immediate halt to the drills and to any hostile activities against the South, the Herald reported.
"We believe the North's maritime firing is a planned provocation and an attempt to test our military's determination to defend the Northern Limit Line and to get an upper hand in South-North relations," South Korea's Min-seok was quoted as saying.
The White House called the North's move "dangerous and provocative."
"We remain steadfast in our commitment [to] the defense of our allies and remain in close coordination with both the Republic of Korea and Japan," White House National Security Council spokesman Jonathan Lalley said.
The Russian Foreign Ministry also released a statement, saying, "We are worried about the mutual toughening of rhetoric, including the declaration by North Korea that it could conduct a new nuclear test."
However, Moscow added that the U.S. should not conduct military exercises with South Korea.
The western sea boundary is poorly marked and has witnessed several violent skirmishes between the North and the South. Four years ago, 46 sailors died after a South Korean warship sank in the area allegedly after a torpedo attack by the North, which denied the charge.