North Korea launched two ballistic missiles on Sunday morning, according South Korea's military and Japan's coast guard.

"Our military has strengthened surveillance and vigilance while closely cooperating with the United States and maintaining a full readiness posturem" South Korea's Joint Chiefs of Staff said.

Japan's defense ministry added that one of the missiles covered a distance of 500 km (310 miles) and flew at an altitude of 550 km.

Japanese public broadcaster NHK also reported that it landed in the Sea of Japan but outside of Japan's exclusive economic zone (EEZ), citing unnamed government sources.

Advanced technologies in development

The Sunday missile launches comes just three days after North Korea said it tested a "high-thrust solid-fuel motor" for a new strategic weapon.

All of North Korea's currently-known intercontinental ballistic missiles (ICBMs) use liquid fuel.

North Korea has ramped up missile tests this year amid large-scale American and South Korean joint training exercises off the Korean peninsula. Pyongyang views these drills as a rehearsal for invasion.

Some of North Korea's recently-tested missiles included mock nuclear warheads.

North Korean leader Kim Jong Un declared he wanted to build the world's most powerful nuclear force after the launch of the Hawsong-17 "monster" missile last month.