Seoul, South Korea (CNN) Clicking through a series of photo slides that depict recent North Korean missile tests, South Korean military defense expert Choi Kang reacts in a tone suggesting he's both fearful and impressed.

"This is really imaginative or creative thinking of using missiles," said Choi, a former director of South Korea's National Security Council and now vice president of the Asan Institute for Policy Studies, a prominent conservative think tank in Seoul.

Experts say the latest missile tests by Kim Jong Un's regime show Pyongyang is, for the first time, actively using testing weapons to target weak points in the advanced missile defense system that protects the US, Japan and South Korea.

Before resuming ballistic missile testing in May, North Korea hadn't test-fired any missiles since November 2017. That pause was a crucial factor in helping create the right conditions for the first meeting between US President Donald Trump and North Korean leader Kim Jong Un in June 2018.

Trump and Kim have met on two other occasions since then, but little progress appears to have been made between the two sides. Trump has played down the significance of North Korea's recent uptick in missile tests, highlighting the fact that Kim only agreed to stop testing longer-range missiles and nuclear bombs. The launches, however, do violate UN Security Council resolutions and threaten South Korea and Japan.

"Kim Jong Un has been, you know, pretty straight with me, I think," President Trump told reporters on August 23. "And we're going to see what's going on, see what's happening. He likes testing missiles, but we never restricted short-range missiles, we'll see what happens. Many nations test those missiles."

However, experts worry that these weapons tests show Pyongyang is much further along in its weapons development than previously thought. The missiles can theoretically be activated at shorter notice and fly faster than their predecessors.

Some warn these new abilities could be applied to longer-range missiles that can reach the US mainland.

"It seems to me that North Korea has a very, very strong indigenous missile capability and ... (is able) to deploy all the missiles in a very short period time," Choi said.

Targeting the gap

North Korea had appeared to put its weapons program on hold for 17 months to allow for negotiations, but the hiatus was broken on May 4 when it tested a new weapons system.

The purpose of the launch, North Korean state media reported, was to "inspect the operating ability and the accuracy of striking duty performance of large-caliber long-range multiple rocket launchers and tactical guided weapons."

That alone was enough to prompt concern, but the most telling series of tests came just days apart in July.

Then, North Korea fired a series of short-range missiles at altitudes mostly between 25 kilometers (16 miles) and 50 kilometers (31 miles) at various distances between 220 kilometers (137 miles) and 600 kilometers (373 miles) from multiple launch sites.

North Korea suggested the US and South Korea had provoked the tests by going ahead with joint military exercises, which mostly consisted of computer simulations. Pyongyang was also angered by Seoul's acquisition of stealth F-35 fighter jets.

The altitudes of the missile's tested concern Choi and other experts because they indicate that North Korea appears to be targeting a gap in two missile defense systems -- the Patriot missile defense system and the Terminal High Altitude Area Defense (THAAD).

The THAAD system targets missiles in the altitude range of 50 to 150 kilometers (31 to 93 miles), while the Patriot system covers 30 kilometers (19 miles) and below, according to Choi. South Korea is developing something to cover the gap.

Kim Dong-yub, an analyst at the Institute for Far Eastern Studies at Kyungnam University in Seoul, said the missiles tested could evade South Korean missile defense systems because at those altitudes they would fly too high for the Patriot batteries to target and too low for THAAD to easily intercept.

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Analysts in South Korea believe that North Korea tested at least three types of new weapons: a large-caliber multiple launch guided rocket system, a North Korean equivalent of a Russian Iskander and a "new weapon" tested on August 10 which flew about 400 kilometers.

It's unclear if the recent missiles that North Korea has tested are designed to carry nuclear warheads or other weapons.

"Nuclear or not ... these are missiles that do threaten our two most important allies and our principal allies in northeast Asia," said retired US Army Gen. Vincent Brooks, a former commander of US Forces Korea. "This has to be something that the United States is drawn into addressing."

Brooks commanded the nearly 650,000 US and South Korean troops that made up ROK-U.S. Combined Forces Command, from April 2016 until November 2018, one of the most active periods on the Korean Peninsula in recent memory.

The strained relationship culminated in North Korea testing an intercontinental ballistic missiles on November 28, 2017, that Pyongyang said could reach the US mainland. After that missile launch, Brooks began cautiously counting the days until the next test. He continued doing so after retiring and watching developments on the Korean Peninsula from his home in Austin, Texas. His count stopped at 520 days on May 4.

While analysts fear North Korea's recent military enhancements, Brooks expressed confidence in the alliance's ability to defend the peninsula, though he declined to reveal specific defense capabilities. He also said the tests make it easier for the US and its allies "to understand the capabilities" of Pyongyang's weapons systems.

"I am certain that the military commands there in Korea and beyond, are thinking through what exactly we are able to do to this," Brooks said. While his count of provocation-free days is on hold for now, Brooks is still hopeful for a peaceful resolution.

"I yearn for the days when Korea can be free from external influences that prevent them from determining their own way; I yearn for the days when Korea can be unified, and I want them to achieve that and I'll be honest that that's where my heart is," he said.