The North's military said it fired 10 rocket artillery rounds after a South Korean navy ship violated the sea border and fired warning shots "on the pretext of tracking down an unidentified ship."
South Korea also scrambled fighter jets when a group of about 10 North Korean military aircraft flew close to their heavily fortified border, and North Korea fired some 560 rounds of artillery.
South Korea's National Security Council condemned the North for escalating tension.
The US military said it was closely consulting with allies and partners following the launches.
The Japan Coast Guard announced at 6:04 AM and 6:19 AM JST that North Korea had launched possible ballistic missiles toward Japan, the government announced an emergency meeting.
Japanese Prime Minister Fumio Kishida called North Korea's actions "barbaric," and said the government would continue to gather and analyze information.
South Korea's Yonhap news agency reported on Saturday North Korea may be preparing to test a submarine-launched ballistic missile (SLBM), citing the South's military.
Iran reportedly has around 20 types of ballistic missiles, some of which are copies of missiles developed in North Korea, or have origins in Russian and Chinese missiles.
Images of the Sinpo South Shipyard suggested that North Korea is preparing to launch a new vessel designed to carry missiles.
The US Defense Department announced that hypersonic weapons and defense development are among its top priorities for the next year.