By Gawon Bae, Junko Ogura and Yoonjung Seo, CNN
(CNN) -- North Korea's suspected launch of its most powerful intercontinental ballistic missile (ICBM) failed on Thursday, according to a South Korean government source, as Pyongyang intensified its battery of missile tests against a backdrop of US and South Korean military drills that had been scheduled to end on Friday.
However, within hours of the presumed failed test, Washington and Seoul agreed to extend those large-scale exercises to an unknown date, according to a statement from the South Korean Air Force, which said "it was necessary to demonstrate a solid combined defense posture of the bilateral alliance under the current security crisis, heightened by North Korea's provocations."
The joint exercises, named "Vigilant Storm," began on Monday and involve 240 aircraft and "thousands of service members" from both countries, according to the US Defense Department.
North Korea had objected to those drills in statements issued this week, before it ramped up tensions on the peninsula with a barrage of weapons tests on Wednesday and Thursday.
Presumed ICBM test
The suspected ICBM was launched from the west coast of North Korea at around 7:39 a.m. local time, and flew about 750 kilometers (466 miles) before falling into the Sea of Japan, also known as the East Sea, east of the Korean Peninsula, Japan's Defense Ministry said.
The South Korean government source said officials suspect it was a Hwasong-17, North Korea's most advanced intercontinental ballistic missile that was first successfully tested on March 24.
That launch set a new standard for Pyongyang, recording the highest altitude and longest duration of any North Korean missile ever tested. The missile hit a maximum altitude of 6,248.5 kilometers (3,905 miles) and flew a distance of 1,090 kilometers (681 miles), according to a report from the Korean Central News Agency at the time. The flight time was 68 minutes, the report added.
However, a South Korean government source told CNN that officials believe the missile fired Thursday only succeeded in separating at the second stage, and seems to have failed after that, falling into the sea between the Korean Peninsula and Japan. Thursday's launch reached a maximum altitude of about 2,000 kilometers (1,242 miles), according to Japan's Defense Ministry -- less than a third of the record height set in March.
In Japan, the presumed ICMB launch triggered warnings to take shelter in the northern Miyagi, Yamagata and Niigata prefectures, where the Japanese Prime Minister's office initially said it was expected to fly over. Japan's Defense Ministry later evaluated that the missile did not cross over Japan.
North Korea followed the presumed ICBM test Thursday with two short-range ballistic missile launches, according to South Korea and Japan.
In a statement Thursday, South Korea's Joint Chiefs of Staff (JCS) said Pyongyang's repeated launches "of ballistic missiles are a serious provocation that harms the peace and stability of not only the Korean Peninsula but also the international community."
Thursday's tests come just hours before US Defense Secretary Lloyd Austin is due to meet his South Korean counterpart Lee Jong-sup at the Pentagon.
US National Security Council spokesperson Adrienne Watson said in a statement Thursday that Washington "strongly condemns" North Korea's ballistic missile tests, saying they were a "flagrant violation of multiple UN Security Council resolutions and needlessly raises tensions and risks destabilizing the security situation in the region."
North Korea's missiles
The Hwasong-17 has been described by North Korean state media as a "powerful nuclear war deterrent."
It could, at least theoretically, put the entire US mainland in range of a North Korean nuclear warhead, but there's a lot of unknowns about the missile's capability to deliver a nuclear payload on target.
It is, however, big enough to carry a nuclear weapon, or possibly several nuclear weapons, according to experts.
Thursday's launches take the count of North Korean missile tests to at least 30 so far this year, according to a CNN tally -- though the count of individual missiles is far higher.
The weapons fired include both cruise missiles and ballistic missiles, the latter of which have formed the large majority of North Korea's tests this year.
There are substantial differences between these two types of missiles.
A ballistic missile is launched using a rocket or rockets, then travels outside of Earth atmosphere, gliding in space before reentry and then descending powered only by gravity to its target.
A cruise missile is powered by a jet engine, stays inside Earth's atmosphere during its flight and is maneuverable with control surfaces similar to an airplane's.
Cruise missiles have smaller payloads than ballistic missiles, so would require a smaller nuclear warhead than a missile designed to hit the mainland United States, such as an intercontinental ballistic missile.
United Nations Security Council resolutions ban North Korea from testing ballistic missiles, but no such restriction applies to the testing of cruise missiles.
North Korea's ability to deploy a nuclear warhead on any kind of missile is unproven.
Record daily total of short-range missiles
On Wednesday, North Korea launched at least 23 short-range missiles of varying types to the east and west of the Korean Peninsula, according to the South Korean Defense Ministry.
It was the highest number of North Korean short-range missiles fired in a single day, and included a ballistic missile that landed close to South Korean territorial waters for the first time since the division of Korea, according to the JCS.
That missile hit international waters 167 kilometers (104 miles) northwest of South Korea's Ulleung island, about 26 kilometers south of the Northern Limit Line (NLL) -- the de facto inter-Korean maritime border that North Korea does not recognize.
Seoul responded Wednesday by launching three air-to-surface missiles from F-15K and KF-16 fighter jets, targeting an area the same distance north of the NLL.
North Korea is launching missiles at an "unprecedentedly high frequency," Japanese Prime Minister Fumio Kishida told reporters on Wednesday.
US Ambassador to the UN Linda Thomas-Greenfield condemned North Korea's unprecedented missile launch, telling CNN the UN would be "putting pressure" on China and Russia to improve and enhance such sanctions.
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