North Korea fired two missiles into the sea on Wednesday, a day after Washington and Pyongyang announced they would resume stalled nuclear talks later this week.
One of the projectiles fell into waters within Japan's exclusive economic zone, Tokyo said, and although the type of weapon was not clear, previous tests have been identified as short-range ballistic missiles.
The South's Joint Chiefs of Staff said the projectiles were fired from Wonsan into the East Sea (Sea of Japan).
"Our military is monitoring the situation for additional launches and maintaining a readiness posture," they said in a statement, without providing further details.
Japanese Prime Minister Shinzo Abe said Tokyo would hold a meeting of the National Security Council to decide how to respond to the firing.
"The launching of ballistic missiles violates UN Security Council resolutions and we strongly protest and strongly condemn it," Abe told reporters.
The launch came a day after the North's Vice Foreign Minister Choe Son Hui said Pyongyang had agreed to hold working-level talks with Washington later this week.
The two sides will have "preliminary contact" on Friday and hold negotiations the following day, Choe said in a statement carried by the official Korean Central News Agency.
US State Department spokeswoman Morgan Ortagus later confirmed the talks, which she said would happen "within the next week".
"It seems North Korea wants to make its negotiating position quite clear before talks even begin," Harry Kazianis of the Center for the National Interest in Washington said after Wednesday's launch.
"Pyongyang seems set to push Washington to back off from past demands of full denuclearisation for what are only promises of sanctions relief," he added.
- Deadlocked talks -
It is not the first time the North has followed up an offer of talks with a weapons test.
Pyongyang tested what it called a "super-large" rocket launcher last month just hours after Choe released a statement saying that the North was willing to resume working-level talks with Washington.
Negotiations between the two have been deadlocked since a second summit between the North's leader Kim Jong Un and US President Donald Trump in February ended without a deal.
The two agreed to restart dialogue during an impromptu meeting at the Demilitarised Zone dividing the two Koreas in June, but the North's anger at a US refusal to cancel joint military deals with South Korea put the process on hold.
Pyongyang also carried out several weapons tests since the meeting that have been downplayed by Trump, who dismissed them as "small" and insisted his personal ties with Kim remained good.
Relations thawed last month after Trump fired his hawkish national security advisor John Bolton, who Pyongyang had repeatedly denounced as a warmonger.
North Korea's chief negotiator responded positively to Trump's suggestion that the two sides try a "new method" of approaching their discussions.
Trump had criticised Bolton's suggestion of the "Libyan model" for North Korea, a reference to a denuclearisation deal with the African nation's former dictator Moamer Kadhafi -- who was killed after being deposed in 2011.