Seoul tested a domestically built submarine-launched ballistic missile hours after rival North Korea fired two ballistic missiles toward the sea.
Nuclear envoys from Japan, South Korea and the US hold talks a day after Pyongyang tested a long-range missile.
The DPRK fired its highest-ever intercontinental ballistic missile reaching an altitude of 4,475 kilometres (2,781 miles).
North Korea on Saturday conducted another successful test of an intercontinental ballistic missile that proved its ability to strike the US mainland, drawing a sharp warning from Trump and a rebuke from China.
China, Pyongyang's main economic and diplomatic ally, opposes any military intervention and calls for a resolution through dialogue.
South Korea and Japan called emergency meetings of top officials after the missile launch off North Korea's east coast.
US warned not to "disregard the reality that its mainland and Pacific operation region are in the DPRK's range for a strike," North's official news agency quotes Kim Jong-un.
The latest missile test is yet another act of defiance by the reclusive state in the face of intense pressure from the US and Pyongang's main ally, China.
The missile exploded within a few seconds of its launch and came hours before US Vice President Mike Pence reached South Korea for talks on how to curb Pyongyang's weapons programme.
Iran confirmed earlier reports that it had tested a new missile on Sunday.
South Korean Defence Ministry says second test of North Korea mid-range 'Musudan' missile test has failed.
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