South Korea announced a provocative series of front-line military exercises on the island shelled by North Korea last week, but later postponed them.
Monday's move was taken as sign of disarray hours after the president vowed to get tough on the North.
Similar live-fire manoeuvres by South Korean troops last week triggered the North's bombardment of Yeonpyeong Island that killed four people and drew return fire in a clash that set the region on edge.
The new drills originally planned for Tuesday could have had even higher stakes: South Korean and American warships are currently engaged in separate military exercises in waters to the south.
Officials at the Joint Chiefs of Staff said the latest drills were postponed after the marine unit on the island mistakenly announced them without getting final approval from higher military authorities. The cancellation had nothing to do with North Korea, and the drills will take place later, one said.
Pyongyang had warned last week that it would consider any South Korean drills off Yeonpyeong Island a deliberate provocation and territorial violation, and urged Seoul to call off last week's exercises. The artillery attack came after South Korea went ahead with its drills.
Earlier on Monday, President Lee Myung-bak gave his first address to the nation in nearly a week, taking responsibility for failing to protect his citizens, expressing outrage at the North's "ruthlessness" and vowing tough consequences for any future aggression.
Mr Lee has come under criticism for what opponents have called lapses in South Korea's response to the attack just eight months after the sinking of a South Korean warship in nearby waters that killed 46 sailors.
Hours after his speech, authorities on Yeonpyeong Island announced new live-fire drills for Tuesday morning, warning residents by loudspeaker to take shelter in underground bunkers well in advance. Another announcement later said there would be no exercise.
Meanwhile, a nuclear-powered US super-carrier and a South Korean destroyer carried out joint military exercises in the waters south of the island in a united show of force by the allies.