"The United States strongly condemns [North Korea's] ballistic missile launches tonight, which violate UN Security Council Resolutions explicitly prohibiting North Korea's launches using ballistic missile technology," State Dept. acting spokesman Mark Toner said.
"U.S. Strategic Command systems detected and tracked what we assess was a North Korean missile launch at 4:34 p.m. CST," the command said in a statement, adding "the North American Aerospace Defense Command (NORAD) determined the missile launch from North Korea did not pose a threat to North America."
The South Korean military confirmed four missiles were detected to have flown more than 600 miles and landed in waters within 200 nautical miles of the coast of Japan, an area Japan's government claims as its exclusive economic zone.
It was not immediately clear what type of missiles were launched, the South Korean Ministry of Defense announced at a press briefing on Monday.
The launches are seen as a reaction by North Korea to joint U.S.-South Korea military drills that kicked off on Friday.
Some 3,600 U.S. service members joined the 28,000 U.S. troops already based in South Korea for the annual exercise, dubbed Foal Eagle, according to the Pentagon.
The exercises include ground, air, naval and special operations field exercises, and have been taking place annually for some four decades.
“These exercises are defensive in nature, and they have been carried out regularly, openly and transparently for nearly 40 years,” Pentagon spokesman Navy Capt. Jeff Davis said on Friday.
"Our commitment to the defense of our allies, including the Republic of Korea and Japan, in the face of these threats, remains ironclad," Toner, the State Dept. spokesman, said.
"We remain prepared—and will continue to take steps to increase our readiness—to defend ourselves and our allies from attack, and are prepared to use the full range of capabilities at our disposal against this growing threat."
ABC News' Chad Murray, Luis Martinez and Joohee Cho contributed to this report.