With progress appearing to backslide, the United States' envoy to North Korea asked Pyongyang on Monday to revive stalled negotiations over Kim Jong Un's nuclear program.

Washington wants Kim to give up his nuclear weapons, but he wants a more gradual approach that would prioritize easing sanctions against his country.

Many experts and officials are skeptical North Korea will ever give up the arsenal of warheads and missiles that have become central to its status as a global pariah.

North Korea said Saturday it had performed another "crucial test" at its long-range launch site. This would help toward "reliably restraining and overpowering the nuclear threat of the U.S.," Pak Jong Chon, chief of the general staff of the Korean People’s Army, said according to the state-owned Korean Central News Agency (KCNA).

"In the situation of the acute confrontation, the U.S. and other hostile forces will spend the year-end in peace only when they hold off any words and deeds rattling us," Pak said.

Last year, President Donald Trump and Kim made history by holding the first talks between sitting leaders of the two nations.

But the talks have since stalled, and North Korea has given the U.S. an ultimatum that if Washington does not make concessions by the end of this year, it will pursue an unspecified "new path."

The fiery rhetoric of the past has been revived, and North Korea has promised a "Christmas gift," according to KCNA — leaving analysts to guess at what that might be. One possibility might be the launch of a satellite, which uses much of the same technology of an intercontinental ballistic missile capable of hitting the U.S. and elsewhere.

After arriving in South Korea, Stephen Biegun, the U.S. special envoy for North Korea, appeared to dismiss the North's ultimatum but said the U.S. was available for talks.

"Let me speak directly to our counterparts in North Korea," he said. "It is time for us to do our jobs. Let's get this done. We are here, and you know how to reach us."

Trump and Kim stand at the demarcation line in the demilitarized zone separating the two Koreas in June 2019. Kevin Lamarque / Reuters

He admitted that it had been "a long year and we have not made nearly as much progress as we would have hoped" but vowed that "we will not give up."

Biegun said that "the United States does not have a deadline, we have a goal: To fulfill the commitments the two leaders made during their historic summit in Singapore."

That summit in June 2018 yielded a loose agreement that was widely criticized by analysts because it contained no new concrete pledges. Jeffrey Lewis, an expert on North Korea's nuclear program at the Middlebury Institute of International Studies in California, said that the North's position has not changed.

Kim's goal has been to force the U.S. to accept his country as one that is "ruled by the Kims and armed with the bomb," Lewis tweeted. He said the problem came down to Trump's insistence that total "disarmament precede a change in the relationship."

In 2017, North Korea carried out a flurry of missile and nuclear tests, and Trump and Kim exchanged fierce rhetoric that worried some that the region might be on the brink of war. The summit last year coincided with a period of apparent rapprochement on that front, but recently the language has reverted.

Earlier this month, North Korea's foreign ministry said Trump had relapsed into "the dotage of a dotard" after the president threatened military action. This recalled an insult — meaning old and weak — North Korea leveled at the president two years ago.

Biegun said it was "regrettable" that these statements "have been so hostile and negative and so unnecessary." He said that they "reflect neither the spirit nor the content" of the relationship between negotiators on the ground.