( 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 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 . Less than two hours later , South Korean broadcaster YTN and Yonhap quoted the JCS as saying the North Korean shelling had ceased . Warning fax . The reclusive state took the unusual step of informing its neighbor of live-fire drills close to the Northern Limit Line ( 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 . Nuclear tests . 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 . Last week , the Stalinist state launched two medium-range ballistic missiles into the sea off its east coast Wednesday , violating United Nations resolutions that prohibits 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 . Pyongyang slams South Korea President over nuclear remarks .
North Korean military exercises see shelling into South Korean waters ; South responds with artillery fire . - The United States and Japan hold a joint exercise in the Sea of Japan. North Korea hints at further nuclear tests . Claim comes days after North 's test-firing of medium-range missiles into the sea .