South Korea, the U.S., and Japan on Tuesday started a joint exercise with Aegis destroyers capable of detecting and intercepting North Korea's ballistic missiles.

It is the fourth drill of the kind since last June and takes place separately from ongoing South Korea-U.S. military exercises. A Navy spokesman here said the aim is "to suit the current security situation."

The three countries are increasingly worried about advancement in the North's missile technology, a government official here said.

