Like most North Korean tests, those missiles all fell in the Sea of Japan, which is known as the East Sea in both Koreas.

But Kim Yo Jong, the powerful sister of leader Kim Jong Un, threatened on Monday to go further, saying North Korea's use of the Pacific as a "firing range" would depend on the behaviour of U.S. forces.

"This type of testing would have technical value as well as communicate the credibility of their nuclear deterrent," said Ankit Panda of the U.S.-based Carnegie Endowment for International Peace.

So far North Korea has fired three variants of the Hwasong-12 intermediate range ballistic missiles (IRBMs) over Japan and into the Pacific Ocean. The last such launch, in October 2022, flew a record distance for any North Korean missile.

There have been no reports of damage or casualties from launches over Japan, but international organizations have criticized Pyongyang for conducting such tests with no warning to civil aircraft or ships.

North Korea has never launched an ICBM on anything but a lofted trajectory, which sends missiles high into space rather than on the lower and longer flight paths that they would follow in real use.

Pyongyang says it does this out of concern for the safety of its neighbours.

"This is a concerning threat and a credible one: North Korea likely seeks to technically validate its longer-range missiles through testing into the Northern Pacific, as it has done with the Hwasong-12 in the past," Panda said.

The Hwasong-15 and Hwasong-17 ICBMs are the main candidates for this type of testing, he added.

REENTRY VEHICLES

Officials in South Korea and the United States have said it is unclear if North Korea has perfected the reentry technology that would protect a nuclear warhead during the fiery decent through the atmosphere.

Kim Yo Jong referenced that debate in her Monday statement, disputing claims by some experts that footage shot from Japan showed a reentry vehicle failing in flight.

"We have possessed satisfactory technology and capability and now will focus on increasing the quantity," she wrote.

Full-range tests into the Pacific would allow North Korea to subject ICBM reentry vehicles to atmospheric stresses and aggregate heat loads that would be more realistic compared to highly lofted trajectories, Panda said.

North Korea's ICBM technology is coming of age, and perfecting reentry vehicles would increase the threat and pressure on the United States, Shin Seung-ki, a Research Fellow at Korea Institute for Defense Analyses (KIDA).

"If that technology is successfully implemented through the test, they will be able to attack the U.S. mainland, which is the purpose of their ICBMs," he said.

North Korea can likely receive telemetry from its short-range and lofted missile tests, but it is unclear if they could collect data from long-range weapons tests, said Markus Schiller, a Europe-based missile expert.

"They should be able to collect inflight data as long as the missile is in sight," he said. "As soon as it is out of range, or if it crosses below the horizon, North Korea will be blind."

Schiller said he is not aware of any tracking vessels that North Korea positions along the flight path, and for now it doesn't have data relay satellites.

NUCLEAR TEST?

South Korean officials are not wrong to note that the North's reentry vehicles are unproven, but those assertions also tempt Pyongyang to conduct the tests necessary to prove its capabilities, George William Herbert, an adjunct professor at the Center for Nonproliferation Studies and a missile consultant, said on Twitter.

To make its point North Korea could resort to conducting a full-range test and detonate a live nuclear warhead over the ocean, he said.

In 2017, North Korea's foreign minister suggested leader Kim Jong Un was considering testing “an unprecedented scale hydrogen bomb” over the Pacific in response to U.S. President Donald Trump’s threat to “totally destroy” the country.

"The so-called Juche Bird live weapon test is a fun joke until the day they actually fire it, then will be a major geopolitical incident and radioactive fallout disaster even if 'safely' detonated high over water," Herbert said. "We shouldn’t be encouraging it by disparaging their capability."

North Korea has completed preparations to possibly resume nuclear detonations in the underground tunnels of its nuclear test site for the first time since 2017, according to officials in Seoul and Washington.

With or without an atmospheric test, North Korea would likely conduct multiple full-range ICBM tests, as well use its underground testing to perfect smaller but more powerful nuclear warheads, said Yoji Koda, a former admiral with Japan's Maritime Self Defense Force.

If those two conditions are met, then North Korea will have fully demonstrated its deterrence capability against the United States, he said.