Japanese Prime Minister Shinzo Abe condemned the launches as a "serious challenge" to both Japan and the global community, even though the projectiles did not land inside Japanese territorial waters. "We will increase our vigilance to preserve the safety and assets of the Japanese people", he told reporters.
The missiles tested on Thursday appear to be North Korea's KN-25, a solid-fuel rocket that is created to fired off in rapid succession from a mobile launcher that typically holds about four tubes.
US -led diplomacy on ending the North Korean nuclear crisis has remained stalled for months. The first incident happened in September, according to South Korea's military.
North Korea fired an "unidentified projectile" today, apparently ramping up its weapons tests while talks with the United States remain deadlocked.
North Korea said Monday its troops conducted artillery firing drills near the sea boundary.
Last month, Kim Jong Un also demanded that South Korea remove what he called its "unpleasant-looking" resort facilities from Mount Kumgang, which was the site of a suspended joint tourism project.
The US and South Korea meanwhile said the murder was orchestrated by Pyongyang. President Donald Trump has called the suspension of those tests a major achievement of his North Korea policy.
"These tests help improve their solid-fuel motors and not only is it good for their short-range systems, it would likely make their long-range missiles quicker and more powerful", said Melissa Hanham, a weapons expert and deputy director of the Open Nuclear Network.
The launches were an apparent attempt by North Korea to pressure the United States to make concessions in their stalemated nuclear diplomacy.
In recent weeks, high-level North Korean officials have issued statements via state media saying their country is not interested in diplomacy with the US unless Washington abandons hostile policies toward the North.
North Korea demands the lifting of sanctions and the abandonment of joint military drills by the US and South Korea in exchange for scaling back its nuclear weapons program.
Earlier this month, a North Korean official said the United States must first drop its "hostile policy" in order to restart stalled denuclearization talks.
President Trump has met three times with Kim Jong Un in hopes of sealing a potentially historic denuclearization deal.