South Korea's armed forces on Monday began a live-fire artillery exercise on an island near the disputed maritime border despite threats of retaliation from North Korea, a media report said.
A Defence Ministry spokesman said the drills would last less than two hours, South Korea's Yonhap News Agency reported.
An official from the South's Joint Chiefs of Staff said the exercise began at around 2:30 pm (0530 GMT), dpa reported.
Pyongyang threatened to respond with "unpredictable self-defensive blows" to the exercises at Yeonpyeong Island, in the Yellow Sea near the disputed maritime border took place.
On November 23, North Korean artillery fire killed two marines and two civilians on the island.
A military source said North Korea had been preparing its weapons in the vicinity including removing gun covers and forward deploying some artillery.
Warnings were broadcast earlier to residents on Yeonpyeong Island to move into shelters.
A Joint Chiefs of Staff official said that South Korean forces were ready to deliver an "immediate and strong" response if North Korea staged any provocations.
Yonhap reported that the navy had send two destroyers into the Yellow Sea to deal with any North Korean provocations and fighter jets were also on standby on the West coast.
The US has defended Seoul's right to hold the exercises while Russia and China have urged restraint.
A spokesman for the Japanese government on Monday morning called on North Korea not to take "provocative action" over the planned drills.
Japanese Prime Minister Naoto Kan ordered his ministers to be ready for possible fresh clashes between the two Koreas.
The United Nations Security Council has failed to reach a consensus on the issue.
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