Seoul and Tokyo CNN  — 

North Korea’s suspected launch of its most powerful intercontinental ballistic missile (ICBM) failed on Thursday morning, 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 until Saturday, according to a statement from the Pentagon.

Earlier Thursday, the South Korean Air Force announced that the two countries had agreed to extend the “Vigilant Storm” military exercises without specifying an end date, saying “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 involved 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.

On Thursday, after the failed suspected launch of the ICBM, North Korea called the extension of the drills a “very dangerous and false choice,” Secretary of Workers’ Party of Korea Central Committee Pak Jong Chon said in a statement published by state media KCNA.

Later on Thursday evening, shortly before US Defense Secretary Lloyd Austin met with his South Korean counterpart Lee Jong-sup at the Pentagon, North Korea fired three short-range ballistic missiles into the waters off its east coast, South Korea’s Joint Chiefs of Staff (JCS) said.

“Our military has strengthened the surveillance and vigilance and is maintaining a fully readiness posture while closely cooperating with the US,” JCS said.

Austin later condemned North Korea’s “irresponsible and reckless activities” during a joint press conference with Lee.

“We’ve said before these kinds of activities are destabilizing to the region potentially. So we call on them to cease that type of activity and to begin to engage in serious dialogue,” Austin said.

Thursday morning’s 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 Hwasong-17 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 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.”

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.”

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 mark the 30th day this year that North Korean has tested missiles, 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.

US Ambassador to the UN Linda Thomas-Greenfield announced Thursday there will a United Nations Security Council meeting on Friday to discuss North Korea’s recent missile launches.

In an interview on MSNBC, Thomas-Greenfield condemned North Korea’s actions and called for Beijing to join in that condemnation and efforts to hold Pyongyang accountable.

“These are resolutions…that we and the Chinese and others worked together to put on the books against the DPRK,” she said, using the acronym for North Korea’s official name: the Democratic People’s Republic of Korea.

The three short-range ballistic missiles fired by North Korea on Thursday evening had a flight range of about 490 kilometers (304 miles) with an altitude of 130 kilometers (81 miles), according to South Korea’s JCS.

The missiles had a top speed of about Mach 6, which means that they traveled six times the speed of sound, JCS said.

The intelligence authorities of South Korea and the US are analyzing further details, JCS said.

JCS said South Korea and the US spoke about the situation shortly after North Korea launched the missiles, and agreed to “further strengthen” combined defense posture against any North Korean provocations.

“Continuous ballistic missile launches by North Korea is a serious act of provocation that harms peace and stability of the Korean Peninsula as well as the international community,” JCS said, urging North Korea to immediately stop firing 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.