US President Donald Trump has reiterated his confidence a deal will be reached with North Korea as the South called on its neighbour to "stop acts that escalate military tension on the Korean Peninsula".
North Korea fired several "unidentified short-range projectiles" into the sea off its east coast on Saturday. South Korean military initially described it as a missile launch but subsequently gave it a more vague description.
The latest firing came after the North's test of what it called a tactical guided weapons system in April.
In a Twitter message on Saturday morning, Trump said he was still confident he could reach a deal with North Korean leader Kim Jong-un.
"I believe that Kim Jong Un fully realizes the great economic potential of North Korea, & will do nothing to interfere or end it," Trump wrote. "He also knows that I am with him & does not want to break his promise to me. Deal will happen!"
Talks stalled after a second summit between Kim and Trump in Hanoi in February failed to produce a deal to end Pyongyang's nuclear program in return for sanctions relief.
Analysts suspect the flurry of military activity by Pyongyang was an attempt to exert pressure on the US to give ground in negotiations.
South Korea's presidency urged North Korea to refrain from further action in one of the most stiffly-worded statements since the two Koreas embarked on reconciliation efforts early last year.
"We are very concerned about the North's latest action," South Korea's presidential spokeswoman said in the statement, adding that it violates an inter-Korean military agreement.
"We expect North Korea to actively join efforts towards the fast resumption of denuclearisation talks," she said, after a meeting attended by the country's defence minister, presidential security advisers, and intelligence chief.
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The projectiles, fired from the east coast city of Wonsan around 9am local time flew about 70 to 200 kilometres in a northeasterly direction, South Korea's Office of the Joint Chiefs of Staff said in a statement.
The South Korean military said it was conducting joint analysis with the US of the latest launches. Experts say the projectiles appeared to come from multiple rocket launchers, and were not ballistic missiles.
The North's last missile launch was in November 2017, when it tested an intercontinental ballistic missile (ICBM). Soon after that missile test, the North declared that its nuclear force was complete, after which Pyongyang extended an olive branch to the South and the US.
But, on Tuesday, North Korea's vice foreign minister warned that the US would face "undesired consequences" if it fails to present a new position in denuclearisation talks by the end of the year.
Trump raised the issue of North Korea during a telephone call with Russian President Vladimir Putin on Friday.
White House Press Secretary Sarah Sanders said Trump told Putin several times "the need and importance of Russia stepping up and continuing to put pressure on North Korea to denuclearise".
During a summit with Putin in late April, North Korea's Kim said peace and security on the Korean peninsula depended on the US, warning that a state of hostility could easily return, according to North Korean media.