South Korea is reportedly searching for four North Korean submarines that disappeared after leaving their bases on the tense peninsula.

A military source quoted in a South Korean newspaper says up to four North Korean submarines slipped out of port in recent days and have so far avoided detection.

The source was also quoted as saying that Pyongyang has stepped up submarine infiltration drills as the weather has warmed.

Seoul is now on alert for a possible strike against a South Korean naval ship.

The South accuses the North of using a midget submarine to sink the corvette the Cheonan two years ago, which left 46 South Korean sailors dead.