The launches are the latest in a string of ever more sophisticated tests by North Korea, which has fired off cruise missiles, tactical rockets and hypersonic weapons in recent months, in what the nuclear-armed country says is a drive to upgrade its defences.
Seoul and Washington have accused North Korea of sending arms to Russia, which would violate rafts of UN sanctions on both countries, with experts saying the recent spate of testing may be of weapons destined for use on battlefields in Ukraine.
Seoul's military said Friday it had detected the launch of what it described as "several flying objects presumed to be short-range ballistic missiles" from North Korea's eastern Wonsan area into waters off its coast.
The missiles travelled around 300 kilometres (186 miles), the Joint Chiefs of Staff said, adding that the military had "strengthened vigilance and surveillance in preparation for additional launches" and was sharing information with allies Washington and Tokyo.
The launches came just hours after Kim's sister Kim Yo Jong accused Seoul and Washington of "misleading the public opinion" on the issue with their repeated accusations that Pyongyang is sending weapons to Moscow for use in Ukraine.
They also come as Russian leader Vladimir Putin was in China on Friday, the final day of a visit aiming to promote crucial trade with Beijing -- North Korea's most important ally -- and win greater support for his war effort in Ukraine.
The tests also came a day after advanced South Korean and US stealth fighters, including Washington's F-22 Raptors, staged joint air combat drills.
Such exercises typically infuriate Pyongyang, which views them as rehearsals for invasion.
- 'Warning message' -
The North has appeared especially sensitive to air drills in the past, with experts noting its air force is the weakest link in its military.
"It appears that this is a counter-military demonstration in response to recent South Korea-US air exercises," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP.
"It also appears to contain a warning message regarding the large-scale South Korea-US joint exercises scheduled for August," Yang said.
Inter-Korean relations are at one of their lowest points in years, with Pyongyang declaring South Korea its "principal enemy".
It has jettisoned agencies dedicated to reunification and threatened war over "even 0.001 mm" of territorial infringement.
The Friday launches are the latest since the North fired a volley of what Seoul said were short-range ballistic missiles off its east coast on April 22.
North Korea is barred by rafts of United Nations sanctions from any tests using ballistic technology, but its key ally Russia used its UN Security Council veto in March to effectively end UN monitoring of violations, for which Pyongyang has specifically thanked Moscow.
Kim Jong Un inspected a new tactical missile weapons system on Tuesday and called for an "epochal change" in war preparations by achieving arsenal production targets.
- 'Likely a hypersonic missile' -
The unusual wording of the South Korean military's statement implies the Friday test was "likely a hypersonic missile", said Hong Min, a senior analyst at the Korea Institute for National Unification in Seoul.
"Hypersonic missiles are not yet stabilised weapons in North Korea," he told AFP, adding this tallied with the launch site in coastal Wonsan.
Despite the short flight trajectory, the launch could have been of "mid-range or longer-class missiles that were fired with an adjusted range for experimental purposes", Hong said.
"There is practically no weapon other than a hypersonic missile that can be described as both ballistic and a 'flying object'," he said.
The military typically describes missiles as "projectiles".
North Korea has long sought to master more advanced hypersonic and solid-fuel technologies, to make its missiles more capable of neutralising South Korean-US missile defence systems and threaten the United States' regional military bases.
Hypersonic missiles are faster and can manoeuvre mid-flight, making them harder to track and intercept, while solid-fuel missiles do not need to be fuelled before launch, making them harder to find and destroy, as well as quicker to use.
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