South Korean officials say the North has assembled troops, ships and aircraft for what is expected to be a large-scale military exercise.

Officials, speaking on condition they not be named, have told South Korean media that MiG-21 fighter jets and about 20 naval vessels have assembled since last week at two bases in the Yellow Sea. Other sources told VOA a simulated coastal landing could begin as soon as Wednesday.

Government officials are quoted as saying they are watching the build-up carefully, but so far there is no reason to believe North Korea is planning anything more than a drill.

Analysts say the North typically conducts military exercises at this time of year, but simultaneous maneuvers involving air, sea and land forces are unusual.

The North has reacted angrily to a series of South Korean and U.S. exercises since North Korean artillery units shelled a South Korean island in November. Admiral Michael Mullen, the U.S. military chief of staff, warned this week that he expects Pyongyang to try again to provoke hostilities.