Japan also reported the launch, with chief cabinet secretary Hirokazu Matsuno condemning it as a threat to peace and security while China urged all sides to preserve for stability.
"We call on relevant sides to keep in mind the overall peace and stability on the peninsula," Chinese foreign ministry spokesman Zhao Lijian told a daily briefing in Beijing when asked about the suspected launch.
Nuclear-armed North Korea had already conducted three other missile tests in less than two weeks before Monday, an unusually rapid series of launches. It said two of them involved single "hypersonic missiles" capable of high speed and manoeuvring after launch, while a test on Friday involved a pair of short-range ballistic missiles fired from train cars.
Monday's launch appeared to involve two SRBMs fired east from Sunan Airfield in Pyongyang, South Korea's Joint Chiefs of Staff (JCS) said in a statement.
North Korea used the airport to test fire the Hwasong-12 intermediate-range ballistic missile (IRBM) in 2017, with leader Kim Jong Un in attendance.
The missiles fired on Monday travelled about 380 km (236 miles) to a maximum altitude of 42 km (26 miles), the JCS said in a statement.
Japanese Defence Minister Nobuo Kishi said the missiles appeared to have landed in the ocean off North Korea's east coast and it was evident that North Korea was using the frequent launches to improve its missile technology.
"The repeated launching of North Korea's ballistic missiles is a grave problem for the international community, including Japan," Kishi told reporters, noting that the tests were a violation of U.N. Security Council resolutions that ban North Korea from all ballistic missile development.
The U.S. military's Indo-Pacific Command said that the launch did not pose an immediate threat to the United States or its allies, but that "these missile launches highlight the destabilising impact of (North Korea's ) illicit weapons programme".
The pace of testing suggested that North Korea had enough missiles to feel comfortable about using them on tests, training, and demonstrations, and they reinforced its deterrent credibility by emphasizing the volume of its missile force, said Mason Richey, a professor at Hankuk University of Foreign Studies in Seoul.
North Korea has not tested its longest-range intercontinental ballistic missiles (ICBMs) or nuclear weapons since 2017, but after denuclearisation talks stalled in 2019, it began testing a range of new SRBM designs.
Many of the latest SRBMs, including the hypersonic missiles, appear designed to evade missile defences. North Korea has also vowed to pursue tactical nuclear weapons, which could allow it to deploy nuclear warheads on SRBMs.
"Every tactical missile launch flaunts how little sanctions have constrained the Kim regime, and how the U.S. ... has failed to make North Korea pay a sufficient cost for short-range missile programme development," Richey said.
'ISOLATING AND STIFLING'
The latest launches have drawn both condemnation and an appeal for dialogue from a U.S. administration that has imposed new sanctions over North Korean missile launches and is pushing for more.
President Joe Biden's administration imposed its first new sanctions on Pyongyang on Wednesday, and called on the U.N. Security Council to blacklist several North Korean individuals and entities. It also repeated calls for North Korea to return to talks aimed at reducing tension and persuading it to surrender its arsenal of nuclear weapons and ballistic missiles.
North Korea has defended the missile tests as its sovereign right to self-defence and accused the United States of intentionally intensifying confrontation with new sanctions.
In a statement before Friday's tests, the North Korean foreign ministry said that although the United States might talk of diplomacy and dialogue, its actions showed it was engrossed in its policy of "isolating and stifling" North Korea.
South Korea's national security council held an emergency meeting after Monday's test, with members stressing it was essential to start dialogue as soon as possible to stop the situation from becoming more strained and to restore stability, the presidential Blue House said in a statement.
The launches came as North Korea, more isolated than ever under self-imposed border closures aimed at preventing a COVID-19 pandemic, appeared to be preparing to open at least some trade across its land border with China.
Freight trains connecting China with North Korea have resumed for the first time since a 2020 coronavirus border lockdown, the Chinese foreign ministry said on Monday
Zhao Tong, a Beijing-based nuclear policy expert at the Carnegie Endowment for International Peace, said North Korea had few reasons to hold back its missile development.
Leader Kim appeared to have little hope of a breakthrough with the United States, and China's sympathy for North Korea and antipathy towards the United States could encourage North Korea to think that China was unlikely to support any effort by the international community to censure it for the tests, he added.
"North Korea may think this is a safe time to advance its missile development," Zhao said.
Last week, China criticised the new U.S. sanctions but also called on all sides to act prudently and engage in dialogue to reduce tensions.
(Reporting by Josh Smith; Additional reporting by Elaine Lies and Sakura Murakami in Tokyo; Yew Lun Tian and Gabriel Crossley in Beijing and Hyonhee Shin in Seoul; Editing by Neil Fullick, Gerry Doyle, Robert Birsel)
By Josh Smith