He made the latest remarks about the moratorium on Tuesday, the last day of a four-day meeting of the Workers’ Party Central Committee, the North’s highest decision-making body. The remarks threatened a major shift in North Korean policy.

He stressed that North Korea “should more actively push forward the project for developing strategic weapons​” now that Washington’s “gangster-like acts” have stymied economic growth.

The unprecedented rapprochement between the United States and North Korea began nearly two years ago, after months of nuclear and ICBM tests and threats from both sides. Two months after Mr. Kim announced his moratorium — saying he had now completed his nuclear force — the two leaders met in Singapore in June 2018 in the first summit meeting between the two countries.

The summit ended with a commitment to “work toward complete denuclearization of the Korean Peninsula,” but the pledge was never understood the same way by the two sides. The administration insisted that meant Mr. Kim would give up his nuclear weapons, his stockpile of fissile material and his missiles, but the North argued that it also meant the United States would withdraw troops and offshore ships and submarines that could launch nuclear weapons.

Subsequent negotiations have failed to close the gap.

Mr. Trump, speaking in Florida on Friday night, cited the Singapore pledge and said he considered Mr. Kim to be a “man of his word.”

“I know that he is sending out certain messages about a Christmas present,” Mr. Trump added, reprising a line he had used before, “and I hope his present is a beautiful vase.”

But Mr. Kim’s words on Tuesday were much harsher. “If the U.S. persists in its hostile policy toward the D.P.R.K., there will never be denuclearization on the Korean Peninsula,” he said, using the initials for the North’s formal name, the Democratic People’s Republic of Korea. “The scope and depth of bolstering our deterrent will be properly coordinated depending on the U.S. future attitude to the D.P.R.K.”