A high-level defector from Kim Jong-un’s regime has sent a letter to President Trump warning that he has been “tricked” into believing the North Korean leader will ever denuclearize and that Washington should instead ramp up a “psychological warfare campaign” aimed at inspiring North Korea’s elites to replace the young dictator from within.

The U.S. should simultaneously impose “all-out sanctions” against Pyongyang and be prepared to carry out a “preemptive strike” against Mr. Kim’s nuclear sites, according to the letter, a copy of which was obtained by The Washington Times.

The warning comes as the White House seeks fresh momentum for Mr. Trump’s historic personal diplomatic outreach to Mr. Kim in the face of a slew of provocative military moves, rhetorical outbursts and more than a dozen ballistic missile tests by Pyongyang in recent months.

“As long as Kim Jong-un remains in power, denuclearization of North Korea is permanently impossible because [Mr. Kim] regards nuclear weapons as the last means to defend his survival,” the defector warned Mr. Trump. “You have stopped Kim Jong-un from launching missiles and conducting nuclear tests, but he is still mounting nuclear threats behind the scenes of dialogue and is attempting to take advantage of the relationship with you.

“The most effective way to resolve the North Korean issue is to conduct psychological warfare operations,” the letter continues. “It can have the same power as a nuclear bomb. It is also an ideal way to get North Koreans to solve their own problems by themselves.”

The White House declined to comment on the defector’s appeal, and Mr. Trump has repeatedly cited the “beautiful letters” Mr. Kim has sent him personally over the past 18 months insisting that the young North Korean leader is sincere in seeking a denuclearization deal.

Two sources verified that the defector’s letter was delivered to two of Mr. Trump’s top North Korea policy advisers: Deputy National Security Adviser Matthew Pottinger and acting National Security Council Asia Director Allison Hooker.

The Times obtained a copy on the condition that it not name the defector because of security concerns. Three sources confirmed that the person worked for many years in high rungs of the North Korean government. The letter writer says he served three decades as an “executive” in the ruling Workers’ Party of North Korea.