(CNN) -- A day after raising the possibility of further nuclear tests, North Korea has engaged in provocative live-fire exercises near the South Korean maritime border, leading to an exchange of fire between the two neighbors.
Semi-official South Korean news agency Yonhap reported Monday that the North had begun the drill just after noon (local time). The Joint Chiefs of Staff (JCS) confirmed that some North Korean ordnance landed in South Korean waters, and that the South responded with fire.
The JCS confirmed that the North Korean offshore military exercise began around 12:15 pm (local time) Monday, and said that "a part of North Korea's shelling reached South Korean side of the NLL (Northern Limit Line) and we (South Korea) responded with K-9 self-propelled guns into the North Korean waters above NLL."
The statement is in line with Yonhap's report that the North fired "several" artillery shells, to which South Korean military responded with self-propelled artillery fire. The South Korean K-9 howitzers have a 24-mile (40-kilometer) range.
It is not clear how many of the shells fired by North Korea reached the Republic's territorial waters. Although there was a lull, North Korean offshore firing seems to have resumed, with Yonhap quoting a resident of Baekryong Island, which is in close proximity to the NLL.
"Some (North Korean) artillery fire landed in (the) southern part of Northern Limit Line -- but in the water," a South Korean Ministry of Defense spokesperson told CNN. "We counter-fired over the Northern Limit Line".
When asked what South Korea fired back at, the MOD spokesman said "We are not shooting at North Korea, just shooting into the sea."
The spokesman declined to answer where South Korea is firing from and if the exchange is still ongoing. The official also refused to confirm if civilians are being evacuated or put into shelters on the front line islands.
The reclusive state took the unusual step of informing its neighbor of live-fire drills close to the NLL in the heavily-militarized western sea, also known as the Yellow Sea. Pyongyang sent a fax early Monday demanding the Republic "control" its vessels in seven sea border areas of the Yellow Sea north of the NLL.
According to Wee Yong Sub, South Korean Defense Ministry vice-spokesperson, the scheduled tests mark the first time -- in recent history, at least -- that the North has announced live-firing exercises above the NLL, which marks the established maritime border between the two Koreas.
"We consider such announcement as a hostile threat and so have activated crisis management operation in case of (military) provocation," he said. "We stress that we are fully prepared for all situations."
He added that there are no immediate signs of nuclear tests being carried out by the North.
North Korea said Sunday it "would not rule out" a new nuclear test as it defended its recent mid-range missile launch that triggered international condemnation.
"(We) would not rule out a new form of a nuclear test aimed at strengthening our nuclear deterrence," Pyongyang's foreign ministry said in a statement carried by the state-run KCNA news agency. "The U.S. had better ponder over this and stop acting rashly."
The statement did not specify what North Korea meant by a "new form" of test.
On Wednesday, the Stalinist state launched two medium-range ballistic missiles into the sea off its east coast, violating United Nations resolutions that prohibit Pyongyang from conducting such tests.
The U.N. Security Council condemned the move and is considering an "appropriate response," the council's president U.N. Ambassador Sylvie Lucas said.
At a briefing Monday, Wee said: "We are fully prepared for all provocation, including North Korea's additional launching of missiles or nuclear test under the close cooperation with the U.S."
The military exercises are the latest provocation by the North, and come after a maritime dispute last week was seemingly swiftly resolved. On Thursday, a North Korean fishing boat was seized following an alleged incursion into South Korean waters, and then returned with its three crew members to the following day.
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