|The South Korean army has been on high alert since North Korean shelling killed four people in November [AFP]
North Korea has launched apparent artillery drills as the top US and South Korean military leaders held talks on the peninsula's security worries following a deadly North Korean artillery strike last month.
The North Korean shells landed in the country's own waters north of Baengnyeong island on the disputed maritime border, a South Korean military official said on Wednesday.
He spoke on condition of anonymity, citing military rules.
Admiral Mike Mullen, the US chairman of the joint chiefs of staff, and his South Korean counterpart, met in the capital, Seoul where they discussed ways to deter North Korean aggression.
The meeting comes amid tensions that have erupted on the divided peninsula after North Korea's November 23 artillery barrage on a South Korean island near the sea border.
'Deliberate and illegal'
Last month's bombardment was "a deliberate and illegal armed attack" which violates the UN Charter and the armistice agreement, the top officers said in a joint statement after the talks.
The attack - the first since the 1950-53 Korean War to target a civilian area - killed two South Korean marines and two construction workers.
Mullen and his counterpart, general Han Min-Koo, said the North's "inhumane actions of killing innocent civilians deserves condemnation".
The two sides said they would continue combined military exercises designed to deter North Korean attacks.
The military chiefs agreed to strengthen joint efforts to deter further provocation and said they would refine existing plans so they can "resolutely respond to further North Korean aggression".
Mullen and Han, in their statement read at a press conference, said it was essential to maintain military readiness "in the face of continued reckless behaviour by North Korea".
Meanwhile, the Obama administration announced that it would send James Steinberg, its No. 2 diplomat, and three other top officials to China next week for talks on North Korea.
The trip comes as Washington presses a reluctant Beijing to do more to rein in its ally, Pyongyang.
Earlier on Tuesday, Steinberg gave a talk to a Washington think tank in which he urged China to press North Korea harder on halting provocative acts.
The trip follows a meeting on Monday between Hillary Clinton, the US secretary of state, and the foreign ministers of Japan and South Korea in which they called on the North to stop "belligerent" behaviour, including last month's attack.
They also rejected China's call for immediate talks with North Korea, saying the North first needed to prove its commitment to peace and return to six-nation nuclear disarmament talks.
Pyongyang recently revealed a uranium enrichment facility, which could give it a second way to make atomic bombs.
Despite that defiance, North Korea has indicated its readiness to resume the negotiations on providing the regime with much-needed aid and other concessions in exchange for disarmament.
On Tuesday, Lee Myung-bak, the South Korean president, promised to transform five islands that lie along the tense maritime border with North Korea into "military fortresses" impervious to North Korean attacks.
Yeonpyeong Island, the island that was attacked in November, is a tiny enclave of military bases and fishing communities along the Koreas' disputed sea border.
The island community remains a ghost town two weeks after the attack, with most of the 1,300 civilian residents seeking refuge on the mainland and many refusing to return.
The two Koreas remain in a technical state of war because their three-year conflict ended in a truce, not a peace treaty.
North Korea disputes the maritime border drawn by UN forces in 1953, and considers the waters around the front-line islands its territory.