The rival Koreas have test-launched ballistic missiles hours apart in a display of military assets amid a faltering diplomatic push to strip North Korea of its nuclear programme.
South Korea's presidential office said it conducted its first underwater-launched ballistic missile test on Wednesday afternoon.
It said a domestically built missile fired from a 3,000-ton submarine flew a set distance before hitting a designated target.
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Meanwhile, North Korea fired two ballistic missiles into the sea off its east coast on Wednesday.
This is North Korea's second weapons test in recent days, after the launch of a new long-range cruise missile.
Japan's Prime Minister Yoshihide Suga has called the recent launch "outrageous" and condemned the action as a threat to the peace and security of the region.
He added: "The government of Japan is determined to further step up our vigilance and surveillance to be prepared for any contingencies."
Japan's coastguard has since said that no ships or aircraft reported any damage from the missiles.
North Korea's state media described the missile as a "strategic weapon of great significance", implying they were developed with the intent to arm them with nuclear warheads.
Many experts say the recent tests suggest North Korea is pushing to bolster its weapons arsenal while applying pressure on President Joe Biden's administration amid a deadlock in nuclear diplomacy between Pyongyang and Washington.
Wednesday's launches came as Chinese foreign minister Wang Yi was in Seoul for meetings with Mr Moon and other senior officials to discuss the stalled nuclear negotiations with the North.
It is unusual for North Korea to make provocative launches when China, its last major ally and biggest aid provider, is engaged in a major diplomatic event.
The US has said the launches do not pose an immediate threat to "US personnel or territory, or to our allies".
Ballistic missile tests contravene United Nations resolutions designed to curb the North's nuclear activities.
Watch: North Korea tests new Cruise missile 'capable of reaching Japan'.
North Korea ended a year-long pause in ballistic tests in March by firing two short-range ballistic missiles into the sea.
Talks between the US and North Korea have stalled since the collapse of a summit between then-President Donald Trump and Mr Kim Jong Un in 2019 when the Americans rejected the North's demand for major sanctions relief in exchange for a partial surrender of its nuclear capabilities.
Mr Kim Jong Un's government has so far rejected the Biden administration's overtures for dialogue, demanding that Washington abandon its "hostile" policies first.
Mr Moon's office said the president told Mr Wang that he appreciates China's role in the international diplomatic push to resolve the North Korean nuclear standoff and asked for Beijing's continuing support.
Mr Wang said Beijing will continue to support the denuclearisation of the Korean peninsula and improved ties between the Koreas.
Cover image: Kim Jong Un watches as missile launches in August 2019 (Picture: KCNA).