North Korea fired several "unidentified short-range projectiles" into the sea off its east coast on Saturday, prompting South Korea to call on its communist neighbour to "stop acts that escalate military tension on the Korean Peninsula".
The South Korean military initially described it as a missile launch, but subsequently gave a more vague description. The latest firing came after the North's test of what it called a tactical guided weapons system in April.
Analysts suspected the flurry of military activity by Pyongyang was an attempt to exert pressure on the United States to give ground in negotiations to end the North's nuclear programme after a summit in February ended in failure.
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 advisors, and intelligence chief.
Talks stalled after a second summit between North Korean leader Kim Jong Un and U.S. President Donald Trump in Hanoi in February failed to produce a deal to end Pyongyang’s nuclear program in return for sanctions relief.
Yang Uk, a senior research fellow at the Korea Defence and Security Forum, described Saturday's action as an expression of the North’s frustration.
"It is a message that it could return to the previous confrontational mode if there is no breakthrough in the stalemate,” said Yang.
The projectiles, fired from the east coast city of Wonsan around 9 a.m. (0000 GMT) flew about 70 kms to 200 kms (44-124 miles) in a north-easterly 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 United States of the latest launches. Experts say the projectiles appeared to be multiple rocket launchers, 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 United States.
But, on Tuesday, North Korea's vice foreign minister warned that the United States would face "undesired consequences" if it fails to present a new position in denuclearization 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. Sanders said Trump told Putin several times “the need and importance of Russia stepping up and continuing to put pressure on North Korea to denuclearize.”
During a summit with Putin in late April, North Korea's Kim said that peace and security on the Korean peninsula depended on the United States, warning that a state of hostility could easily return, according to North Korean media.
"The North wanted to deliver a message on security guarantees to Washington through the mouth of Putin, but the summit fell short of driving change in the U.S. attitude, leading the North to take stronger action today," said Hong Min, a senior researcher of Korea Institute for National Unification.
U.S. Secretary of State Mike Pompeo and South Korean Foreign Minister Kang Kyung-wha agreed to "cautiously respond" to the latest firing and to continue communications during a phone call on Saturday, South Korea's foreign ministry said in a statement.
White House Press Secretary Sarah Sanders said, “We are aware of North Korea’s actions tonight. We will continue to monitor as necessary.”
Pompeo also held talks with Japan's Foreign Minister Taro Kono, and agreed, together with South Korea, to cooperate and share information, the Japanese Foreign Ministry said.
"At this point, we have not confirmed any situation where Japan’s national security would immediately be affected,” Japan’s Defense Ministry said in a statement.