Washington (CNN) A recent spate of North Korean missile launches involving a potential new missile have rankled America's regional allies, but have received little criticism from the Trump administration -- even though they reveal Pyongyang is making technological advances.

North Korea has fired off six missile tests in the last month, most recently on Thursday night ET, launching what a US official described as two short range ballistic missiles.

And while Trump has downplayed the missiles, dismissing them as short range and not a breach of understandings he has with North Korean leader Kim Jong Un, US allies South Korea and Japan could have reason to be concerned. Experts say the new rockets appear to be more accurate and potentially harder for adversaries to hit.

'Marked improvement'

"So far we have seen six missile tests in the last several weeks and those missile tests really continue to show a marked improvement in the North Korean rocket program," CNN military analyst and retired Air Force Col. Cedric Leighton said.

Trump's tendency to minimize the launches is raising concerns that he is tacitly giving North Korea permission to continue testing. "I say it again," Trump told reporters Saturday, "there have been no nuclear tests. The missile tests have all been short-ranged — no ballistic missile test. No long-range missiles."

White House national security adviser John Bolton told VOA Thursday that our allies are "quite concerned about it."

"The latest test of a missile we denominate the KN 23, we think the range could probably hit all of South Korea and parts of Japan," Bolton continued. "That, of course, would endanger our deployed forces as well."

The US has about 28,000 troops stationed in South Korea and another 50,000 in Japan.

Bolton added that the missile launches "violate UN Security Council sanctions," but "don't violate the pledge that Kim Jong Un made to President Trump," while adding that "they are troubling for everybody watching the peninsula."

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Korean media said in a report dated August 17 that Kim himself "guided the test-fire of a new weapon again on Friday morning." The state news agency KCNA claimed that the test showed a "perfect result" and built "confidence" and the missiles are being developed for the sake of national defense and "the major military striking means."

The nation's plan, KCNA said, is to create a "powerful force strong enough to discourage any [other] forces from daring to provoke us."

US officials, including Secretary of State Michael Pompeo, have repeatedly said their goal is North Korea's fully verifiable denuclearization and that Kim is on board with that plan.

On Friday, South Korea urged North Korea to suspend the ongoing series of short-range launches, saying they could escalate military tensions, according to a statement released by the Presidential Blue House following an emergency session of South Korea's National Security Council.

The statement went on to say that the US and South Korea would analyze the latest launches.

Experts see the latest missile as indicative of an increased military capability, saying that the test appeared successful and that the missile displays features that could make it harder for the US military to shoot it down.

Those features include a lower trajectory and in flight maneuverability that could help it evade missile defenses, according to Thomas Karako, the director of the Missile Defense Project at the Center for Strategic and International Studies in Washington.

"That matters because when you fire your interceptor, you have to have an estimate of where (the threatening missile is) going to be," Karako told CNN.

'More and more accurate'

"For them, accuracy is something that they have had difficulties with in the past, but they are getting more and more accurate with each of the missile tests that we have seen," Leighton said.

Karako said that while the missile represents a "significant capability," he said that US troops in South Korea had anticipated this type of threat in recent years and had worked to integrate the Patriot and THAAD missile defense systems on the peninsula to counter the threat, which is seen by some as having similarities with the Russian military's Iskander missiles.

While North Korea has suspended its testing of nuclear weapons and longer range missile launches, such as those that could potentially reach the continental United States, it has persisted in firing off shorter range variants.

Pyongyang has tied the missile launches to an ongoing US-South Korea military exercise that is expected to last through the end of the month.

Trump appeared to confirm that linkage recently, commenting on a letter he had received from North Korean leader Kim Jong Un.

"It was a long letter, much of it complaining about the ridiculous and expensive exercises. It was also a small apology for testing the short range missiles, and that this testing would stop when the exercises end," Trump tweeted earlier this month.

While the US and South Korea have sought to lower the profile of their joint military exercises in order to avoid hindering diplomatic efforts, military officials see such drills as critical to maintaining their readiness.

Trump has in the past lambasted the exercises, referring to them as war games and calling them "ridiculous" and "expensive."

By downplaying the missile tests Trump could be seeking to keep the diplomatic efforts with Kim alive, but there appears to be little tangible progress on denuclearization, despite the letter writing.

"We haven't had really any substantive negotiations at the working level with North Korea since the President met with Kim Jong Un at the Demilitarized Zone" in June, Bolton told VOA.

"We're hoping those talks begin soon. The real issue is whether North Korea will make the clear strategic decision to give up its nuclear weapons and its delivery system," he added.