At least one missile was fired by North Korea into the Korean peninsula and Japan on Tuesday.
The launch was reported by South Korea's Joint Chiefs of Staff, according to Yonhap, who said they were still analyzing what type of missile had been fired, and that it had landed in the East Sea, also known as the Sea of Japan.
Japan's Coast Guard said a second missile may also have been launched, according to Kyodo News agency.
The launch is the latest in a series of weapons tests by Pyongyang, and comes as Seoul and Washington ramp up defense cooperation in the face of soaring tensions with the North.
On Tuesday, they held the first Nuclear Consultative Group meeting in Seoul and announced an American nuclear submarine was making a port visit to Busan for the first time since 1981.
The move had been expected to trigger a strong response from North Korea, which baulks at having U.S. nuclear assets deployed around the Korean peninsula.
The launch also comes less than a week after leader Kim Jong Un personally oversaw the firing of the country's newest intercontinental ballistic missile, the solid-fuel Hwasong-18.
Diplomacy between Pyongyang and Seoul has stalled and Kim has called for ramping up weapons development, including tactical nukes.
In response, Seoul and Washington have staged joint military exercises with advanced stealth jets and U.S. strategic assets.
The launch also comes as Washington confirmed Tuesday that a U.S. soldier is believed to have been detained by North Korea after crossing the heavily fortified border.
