North Korea says it has put its military on high alert, ready to launch operations.
A spokesman for the North's military on October 8 warned the United States of "disastrous consequences" for moving a group of ships, including an aircraft carrier, into a South Korean port.
South Korea's Defense Ministry said a day earlier that the ships were taking part in a routine maritime search-and-rescue exercise and said any criticism by North Korea was "wrong."
An attempt at dialogue between North and South Korea in August led to the reopening of a jointly run factory park that was shut amid high tensions in April.
However, talks have since hit a stalemate.
Based on reporting by Reuters and AFP