North Korea has unveiled for the first time what it says is a nuclear-powered submarine under construction, a weapons system that can pose a major security threat to South Korea and the US.

North Korea fired multiple ballistic missiles on Monday, hours after condemning the South Korean and U.S. militaries for launching drills that Pyongyang called a "dangerous provocative act" that risked accidentally sparking a confrontation.

North Korea fired several ballistic missiles into the sea after South Korean and U.S. troops began their large annual combined drills, Seoul said.