North Korea fired a salvo of short-range ballistic missiles early Thursday, Seoul's military said, hours after Pyongyang sent hundreds of trash-filled balloons across the border to punish South Korea.
South Korea's Joint Chiefs of Staff said it had detected the launch of "what are suspected to be around 10 short-range ballistic missiles," fired into waters east of the Korean peninsula, it said.
The missiles flew around 350 kilometres (217 miles), JCS said, adding that it was analysing the specifics alongside the United States and Japan.
The launch was a "provocation that seriously threatens peace and stability on the Korean Peninsula", it added.
Japan's coastguard and prime minister's office also confirmed the launch, and said they were checking for more information.
On Wednesday, Pyongyang sent balloons full of trash, toilet paper and suspected animal faeces into the South, with Seoul's military slamming Pyongyang for their "low class" actions.
The North had warned over the weekend that it would shower border areas in "mounds of wastepaper and filth" to punish Seoul.
North Korean leader Kim Jong Un's powerful sister Kim Yo Jong mocked the "goblins of liberal democracy" in Seoul for complaining about the balloons, and pledged more could follow.
The salvo of ballistic missiles also comes just days after North Korea's latest attempt to put a spy satellite into orbit ended in a mid-air explosion on Monday.
North Korea said late Monday that the rocket carrying its "Malligyong-1-1" reconnaissance satellite exploded minutes after launch due to a suspected engine problem.
Japanese broadcaster NHK ran footage of what appeared to be a flaming projectile in the night sky, which then exploded into a fireball, saying it had filmed it from northeast China at the same time as the attempted launch.
Putting a reconnaissance satellite into orbit has long been a top priority for Kim's regime, and it claimed to have succeeded in November, after two failed attempts last year.
In a speech released by the official Korean Central News Agency late Wednesday, Kim said the country was undeterred by the recent failed satellite launch.
"Although we failed to achieve the results we had hoped to get in the recent reconnaissance satellite launch, we must never feel scared or dispirited but make still greater efforts," he said.
"It is natural that one learns more and makes greater progress after experiencing failure," he said, according to the transcript of the speech, given at the Academy of Defence Sciences.
Nuclear-armed North Korea is barred by multiple UN resolutions from tests using ballistic technology, and analysts say there is significant technological overlap between space launch capabilities and the development of ballistic missiles.
Monday's launch was widely condemned, including by Seoul, Japan and the United States. The United Nations Security Council is scheduled to meet Friday to discuss that incident.
Early Thursday, North Korea released a statement warning UN Secretary-General Antonio Guterres he might be "the most spiritless and weak-willed secretary-general in the history of the United Nations".
Pyongyang said it had to express "deep concern about the fact that the UNSC is going to convene an open meeting again to call the DPRK's legitimate satellite launch into question," said the statement by Kim Son Gyong, a North Korea foreign ministry official.
For the North, "the launch of a military reconnaissance satellite is an inevitable undertaking for bolstering up the might of self-defence," he added.
Seoul claims Kim received Russian technical assistance for its successful November launch in return for sending containers of weapons to Moscow for use in Ukraine.
