"Our military is monitoring the situation in case of additional launches and maintaining a readiness posture," the South Korean Joint Chiefs of Staff said, according to Yonhap News Agency. Further, the Blue House announced a meeting of the National Security Council would soon follow as a result of the test.
The Japanese Coast Guard said they detected the launch, which did not land in Japan's territory or Exclusive Economic Zone, and warned ships to stay clear of any refuse in the water.
Japanese Defense Minister Takeshi Iwaya said the missile launches are a clear violation of UN resolutions and cannot be ignored. However, US national security adviser John Bolton has previously said the tests did not violate previous agreements made with Pyongyang and US President Donald Trump has dismissed previous tests as unimportant, compared to the longer-range missiles that can threaten the wider region.
Senior US officials also said they were aware of reports of the launch and were consulting with Seoul and Tokyo on the matter.
Saturday's test comes a week after the country's previous weapons test, which saw it fire two short-range ballistic missiles into the Sea of Japan, as it has done numerous times in recent months. The most recent tests have been protest messages by Pyongyang against South Korean participation in war games with the US, but it's unknown what Saturday's test was in response to, if anything.
However, the test does follow a growing row between Seoul and Tokyo that has expanded to encompass not only economic relations, but defense cooperation as well, with the two countries agreeing to share intelligence only through an intermediary power.
Neither Korea gets along well with Japan, as the entire region was colonized for decades by the Japanese Empire and a number of contentious and unresolved issues remain between them.
However, South Korea has announced a large military expansion in recent weeks, which it says is in response to North Korean weapons tests.
"The ministry has decided to concentrate on enhancing defense capabilities to make leading responses to security threats from all directions possible, as well as on building and properly managing a smart military based upon state-of-the-art science technologies," the South Korean Defense Ministry said in an August 14 statement.
