As North Korea fired off a series of missiles in recent months — at least 18 since May — President Trump has repeatedly dismissed their importance as short-range and “very standard” tests. And although he has conceded “there may be a United Nations violation,” the president says any concerns are overblown.

Kim Jong-un, North Korea’s leader, Mr. Trump explained recently, just “likes testing missiles.”

Now, American intelligence officials and outside experts have come to a far different conclusion: that the launchings downplayed by Mr. Trump, including two late last month, have allowed Mr. Kim to test missiles with greater range and maneuverability that could overwhelm American defenses in the region.

Japan’s defense minister, Takeshi Iwaya, told reporters in Tokyo last week that the irregular trajectories of the most recent tests were more evidence of a program designed to defeat the defenses Japan has deployed, with American technology, at sea and on shore.

Mr. Kim’s flattery of Mr. Trump with beguiling letters and episodic meetings offering vague assurances of eventual nuclear disarmament, some outside experts say, are part of what they call the North Korean leader’s strategy of buying time to improve his arsenal despite all the sanctions on North Korea.