North Korea fired two projectiles on Thursday, the South's military said, with nuclear talks between Pyongyang and Washington at a deadlock.
The weapons were launched from South Pyongan province in an easterly direction over the sea, Seoul's Joint Chiefs of Staff said in a statement, without specifying the type of device involved.
"We are maintaining readiness and monitoring in case of additional launches," they added.
It is the latest in a series of launches by the North but the first since October 2, when it fired a sea-launched missile in a provocative move -- a submarine-based missile capability would change the military balance.
The North then walked away from working-level nuclear talks with the US in Sweden, saying it was disappointed at the lack of "new and creative" solutions offered by Washington.
Pyongyang is under multiple sets of international sanctions over its banned nuclear weapon and ballistic missile programmes, which it says it needs to defend against a possible US invasion.
It is demanding the easing of the measures and has repeatedly urged Washington to come forward with a new offer by the end of this year.
On Sunday, the North's state media carried a statement from Kim Yong Chol -- previously the North's counterpart to US Secretary of State Mike Pompeo -- accusing Washington of seeking to "isolate and stifle the DPRK in a more crafty and vicious way than before".
He lauded the "close personal relations" between leader Kim Jong Un and US President Donald Trump, but warned: "There is a limit to everything."
Talks between Pyongyang and Washington have stalled since the two men's Hanoi summit broke down without an agreement in February.
The meeting had been intended to build on their high-profile first summit in Singapore last year, when Mr Kim signed a vague pledge to work towards "denuclearisation of the Korean peninsula".
The latest launch displayed Pyongyang's frustration over the lack of progress in nuclear talks, analysts say.
"The launch is a display of its warning to both Seoul and Washington that it can carry out more military activities unless the US comes up with a 'new method'," said Lim Eul-chul, professor of North Korean studies at Kyungnam University.
Thursday's launch came despite Mr Kim sending a message of condolences to the South's President Moon Jae-in on Wednesday for the death of his mother, who was born in the North.
"From our perspective, the North's behaviour is not rational that it does something like this during the mourning period," Mr Lim said.
"But the launch today shows that the true nature of North Korea is inexplicable in our value system."
Contacts between the North and South have stalled since Hanoi and Pyongyang has repeatedly excoriated Seoul for joint military drills with Washington and not following through on inter-Korean agreements signed last year.
Mr Kim last week inspected the Mount Kumgang complex which once hosted Southern tourists visiting the North -- a symbol of inter-Korean cooperation -- and ordered the demolition of Southern-built facilities.
Pyongyang this week refused Seoul's request for face-to-face meetings to discuss the issue.