North Korea has positioned surface-to-surface missiles in the Yellow Sea, as the US and South Korea carry out joint military exercises.
The planned manoeuvers come days after North Korea shelled the island of Yeonpyong, killing four South Koreans.
Pyongyang has warned of a “brutal military blow” if its territory is violated.
South Korean President Lee Myung-bak met with a visiting Chinese delegation to ask them for help in calming the situation.
China, North Korea’s only major ally, agreed the situation is “worrisome” and said it would strive to stop things from sliding further.
Beijing also called for an emergency meeting of the six governments involved in the North Korean disarmament talks to begin in early December.
However, South Korea said now is not the time for talks.
Earlier the South Korean government called on all journalists to leave Yeonpyong.
The island has been mostly evacuated with those that remain living in protective bunkers.
Washington says the war games are designed to act as a deterrent after Tuesdays shelling, the worst assault on South Korea since the end of the Korean war in 1953.