South Korea's joint chiefs said the projectiles, launched near the eastern port city of Wonsan, were believed to be surface-to-ship cruise missiles.
"We assess that North Korea intended to show off its various missile capabilities, display its precise targeting capability, in the form of armed protests against ships in regard to US Navy carrier strike groups and joint naval drills," Roh Jae-cheon, a spokesman for South Korea's Joint Chiefs of Staffs told reporters.
North Korea has launched 16 missiles in 10 tests so far in 2017, and Thursday's test was the fourth since new South Korean President Moon Jae-in took office in May.
Analysts say each launch, successful or not, improves missile technology and ultimately provides information that helps bring North Korea closer to its goal of building a missile that could reach the US.
The launch comes one day after Moon's government suspended the deployment
of a controversial US missile defense system which had strained relations with China and angered North Korea.
The missiles flew about 200 kilometers (124 miles), South Korea's military said in a statement, adding the US military was undertaking a more detailed analysis.
"Our military has strengthened surveillance and alertness readiness in cases of additional provocation by North Korean military and is maintaining all readiness posture while we are tracking and monitoring related situation," the statement read.
A US defense official confirmed a launch had been detected but told CNN the Pentagon was unlikely to release a statement as these were not ballistic missiles capable of posing a long-range threat.
North Korea warns Japan
North Korean state media made no mention of the reported launches Thursday, but earlier warned Japan not to "gamble on its destiny."
"If Japan is concerned about its security, it should not act (as) a poodle of the US but withdraw its hostile policy toward the DPRK and remove the US military bases for aggression (sic) from its territory," the statement said.
Speaking at a doorstop interview, Japanese Foreign Minister Fumio Kishida condemned North Korea's latest missile launch. "These provocative actions are intolerable," he said.
Analyst: Projectiles could take out carrier
Carl Schuster, a Hawaii Pacific University professor and former director of operations at the US Pacific Command's Joint Intelligence Center, told CNN that this type of missile could, "if they got really lucky," immobilize a US aircraft carrier.
Two US aircraft carriers, the USS Carl Vinson and the USS Ronald Reagan, have recently departed the Korean Peninsula after conducting exercises there in what a US official described as a message to North Korea.
Cruise missiles typically fly in a straight line while a ballistic missile flies miles into the air in an arc to reach its target, he said.
Schuster said North Koreans fire missiles usually for three reasons -- international posturing, domestic propaganda and military training.
"If you really think you're under threat you might want to do a test firing to be ready -- so think of it as both a political statement and a training demonstration," he said.
Fourth missile test in a month
Thursday's launch is the first since the United Nations Security Council unanimously passed new sanctions last week.
The vote came after North Korea fired what it claimed was a new type of ballistic missile from Wonsan that Japanese and South Korean monitors said flew 248 miles (400 kilometers) east of the Korean Peninsula.
The resolution slapped additional sanctions on North Korea and condemned the regime's continued proliferation of its nuclear and ballistic program.
The sanctions also extend a travel ban and asset freeze on high-level North Korean officials and state entities that deal with the program.
China has called on Pyongyang to suspend its testing while calling on the US to stop military exercises on and near the Korean Peninsula, which North Korea sees as a threat to its sovereignty.