(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 neighbors.
The semiofficial South Korean news agency Yonhap reported Monday that the North had begun the drill just after noon. The South Korean Joint Chiefs of Staff confirmed that some North Korean ordnance landed in South Korean waters and that the South responded with fire.
North Korea fired about 500 artillery shells into waters north of the Yellow Sea's so-called Northern Limit Line over a roughly three-hour period Monday afternoon, the Joint Chiefs told Yonhap. After about 100 shells fell south of the Northern Limit Line, South Korea responded by firing about 300 artillery shells into North Korean waters and dispatching fighter jets near the Northern Limit Line.
The Joint Chiefs said the North Korean offshore military exercise began about 12:15 p.m. Monday and said that "a part of North Korea's shelling reached South Korean side of the NLL and (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 the South Korean military responded with self-propelled artillery fire. The South Korean K9 howitzers have a 24-mile (40-kilometer) range.
Although there was a lull, North Korean offshore firing seems to have resumed, with Yonhap quoting a resident of Baekryong Island, which is close to the Northern Limit Line.
"Some (North Korean) artillery fire landed in (the) southern part of Northern Limit Line but in the water," a South Korean Ministry of Defense spokesman said. "We counter-fired over the Northern Limit Line."
When asked what South Korea fired back at, the defense spokesman said, "We are not shooting at North Korea, just shooting into the sea."
The spokesman declined to say where South Korea is firing from and whether the exchange is still ongoing. The official also refused to confirm whether 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 Northern Limit Line in the heavily militarized western sea, known as the Yellow Sea. Pyongyang sent a fax early Monday demanding that the republic "control" its vessels in seven sea border areas of the Yellow Sea north of the Northern Limit Line.
According to Wee Yong-Sub, a vice spokesman for the South Korean Defense Ministry, the scheduled tests mark the first time -- in recent history, at least -- that the North has announced live-firing exercises above the Northern Limit Line, 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 that it "would not rule out" a new nuclear test as it defended its recent midrange 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," said the council's president, Luxembourg Ambassador Sylvie Lucas.
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."
White House National Security Council spokeswoman Caitlin Hayden also denounced North Korea's actions as "dangerous and provocative."
"Its continuous threats and provocations aggravate tensions and further its isolation. We remain steadfast in our commitment (to) the defense of our allies and remain in close coordination with both (South Korea) and Japan."
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 after an alleged incursion into South Korean waters and returned with its three crew members the following day.
And while North Korea often upsets its neighbors by firing various rockets and missiles into the waters surrounding the Korean Peninsula, the country has at times engaged in more deadly military actions.
A multinational 2010 report indicated that the sinking of the South Korean navy warship Cheonan, which killed 46 sailors in the Yellow Sea, was the result of a a North Korean torpedo. Later that year, North Korean artillery attacks on Yeonpyeong Island in the Yellow Sea killed two South Korean marines in what Yonhap called "the first direct artillery attack on South Korean territory since the Korean War ended in an armistice" in 1953.
CNN's Jim Acosta contributed to this report.