South Korea has accused the North of blocking the return of 27 North Koreans who strayed across the border. The North has said Seoul is keeping hostage four North Koreans, from the boatload of 31 which drifted over the sea border in heavy fog on 5 February.
Both North and South have called for talks to ease tensions; the last effort ended without agreement last month.
For its part, North Korea has accused the South of "despicable unethical acts … This cannot be interpreted otherwise than a grave provocation to the DPRK (North Korea)," said a statement attributed to a spokesman for the North's Red Cross.
Of the large-scale military drills being carried out by US and South Korean forces, the North says they are a rehearsal for invasion to topple its government.