North Korea fired two ballistic missiles into the sea on Wednesday and South Korean test-fired a ballistic missile from a submarine in a sign of a growing arms race.
The South's military said the nuclear-armed North had fired "two unidentified ballistic missiles" from its central inland area into the sea off its east coast.
"South Korean and US intelligence agencies are conducting detailed analysis," Seoul's Joint Chiefs of Staff said in a statement without immediately giving details of the missiles' range.
The launch came as Chinese foreign minister Wang Yi visited Seoul for talks with his South Korean counterpart.
Beijing is the North's key diplomatic ally and main partner for trade and aid, although Pyongyang is under a self-imposed blockade after closing its borders early last year to protect itself against the coronavirus pandemic.
Speaking before the news emerged, Wang hoped that all countries would help "peace and stability on the Korean Peninsula", Yonhap news agency reported.
"For example, not only the North, but also other countries are engaging in military activities," he added.
"Having said this, we all have to work together toward the resumption of dialogue."
Wednesday's launches came days after the North's official Korean Central News Agency reported that it had test-fired a new "long-range cruise missile" over the weekend, calling it a "strategic weapon of great significance".
Pictures in the Rodong Sinmun newspaper on Monday showed a missile exiting one of five tubes on a launch vehicle in a ball of flame, and a missile in horizontal flight.
Such a weapon would represent a marked advance in North Korea's weapons technology, analysts said, better able to avoid defence systems to deliver a warhead across the South or Japan -- both of them US allies.
The missiles fired at the weekend travelled 1,500-kilometres (about 930 miles), on two-hour flight paths -- including figure-of-eight patterns -- above North Korea and its territorial waters to hit their targets, according to KCNA.
North Korea is under international sanctions for its nuclear weapons and ballistic missile programmes, which it says it needs to defend itself against a US invasion.
But Pyongyang is not banned from developing cruise missiles, which it has tested previously.
South Korea fires first submarine-launched ballistic missile
Meanwhile, South Korea successfully test-fired a submarine-launched ballistic missile on Wednesday, becoming only the seventh country in the world with the advanced technology and raising the prospect of a regional arms race.
The test, supervised by President Moon Jae-in, came hours after North Korea fired its two ballistic missiles into the sea.
It is a strategic advance for the South, which has been strengthening its military capabilities as it seeks to counter the threat posed by the North, which is under international sanctions for its nuclear weapons and ballistic missile programmes.
"It's extraordinary timing that you have not one but two Koreas testing ballistic missiles on the same day," Yonsei University professor John Delury told AFP.
"It does speak to the fact that there's an arms race in this region that everyone needs to pay attention to."
The South's missile was fired underwater from its newly commissioned submarine Ahn Chang-ho, and flew the planned distance before hitting its target, the presidential Blue House said.
All other countries with proven SLBM capabilities have nuclear weapons of their own.
With the successful tests, South Korea now has "sufficient deterrence to respond to North Korea's provocations at any time", President Moon said, urging the South to continue increasing its weapons programmes to "overwhelm North Korea's asymmetric power".
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