The United States and North Korea will focus on a proposed shipment overseas of some of Pyongyang’s nuclear warheads during preparatory talks for a summit between President Donald Trump and the North’s leader, Kim Jong Un, U.S. officials said Sunday.

The U.S. has demanded that the North ship up to 20 warheads overseas at an early date as part of measures to achieve a “complete, verifiable and irreversible” denuclearization of the country, according to officials. However, reaching a deal is likely to be a challenge given the North’s reluctance to meet such a demand.

According to the U.S. officials, who spoke on condition of anonymity, North Korea has expressed a reluctance to ship all of its nuclear weapons and missiles outside the country.

It is likely that the North has proposed instead to ship in advance certain types of missiles, including intercontinental ballistic missiles capable of hitting the United States.

If U.S. and North Korean officials are unable to reach a deal in the preparatory talks, a decision could be left to the Trump-Kim summit, according to the U.S. officials.

The officials are also expected to discuss how Pyongyang will scrap its weapons-grade plutonium, highly enriched uranium and other nuclear arms-related material, they said.

U.S. national security adviser John Bolton said earlier this month that Washington will seek the dismantlement of North Korea’s nuclear weapons and equipment, as well as its ballistic missile program and chemical and biological weapons.

“I think the implementation of the decision means getting rid of all the nuclear weapons, dismantling them, taking them to Oak Ridge, Tennessee,” Bolton said, referring to the site of a nuclear weapons plant, which stores Libya’s dismantled nuclear weapons equipment.

Skeptics, however, have called this plan unrealistic.

“This is pure fantasy,” said Van Jackson, a North Korea expert and former policy adviser in the U.S. office of the secretary of defense.

He said that while “there’s no way” Kim agrees to relinquish control of all his nuclear warheads or his missile inventory, “there’s a possible future in which he allows some to be removed, as a strategically motivated confidence-building measure, and in that scenario shipping weapons to Oak Ridge seems plausible.”

However, he said, “where they go isn’t the issue for Kim; it’s whether they go, and what percentage.”

But others say even giving a limited number of warheads would be a dangerous gamble for the North.

“The reason is that giving us nuclear weapons would be giving us too much information,” Cheryl Rofer, a former scientist at the United States’ Los Alamos National Laboratory, wrote on Twitter earlier this month. “Those nuclear weapons could be disassembled to show their design,” possibly giving the United States clues to how far along the North’s program is and how it could be confronted.

“Nuclear weapon design is one of the most closely held secrets of any nation,” Rofer wrote. “Why should North Korea be different?”