A top U.S. government adviser who has in the past been granted unprecedented access to North Korea’s nuclear weapons program is warning that the “quick” denuclearization process sought by the Trump administration could, in fact, take far longer, possibly up to 10 years, according to an analysis by experts.

Siegfried Hecker, a former director of the Los Alamos weapons laboratory in New Mexico who is currently a professor at Stanford University in California, argues in a report released late Monday that the best the U.S. can hope for is a phased approach to dismantling the North’s nuclear weapons program that targets the most dangerous parts first.

But such an approach would be at odds with U.S. President Donald Trump’s demand that the North immediately and completely relinquish his arsenal without American concessions in return.

Trump is likely to meet North Korean leader Kim Jong Un in Singapore on June 12 for a highly anticipated summit to discuss the North’s denuclearization. Frantic diplomatic efforts to reach an agreement on that date saw a top North Korean official widely known as Kim’s right-hand man, Kim Yong Chol, depart Beijing on Wednesday for a meeting with U.S. Secretary of State Mike Pompeo this week in New York. He would be the highest-level North Korean official to travel to the U.S. since 2000. The two could discuss their views on denuclearization.

The White House has repeatedly said it will seek the “complete, verifiable, irreversible dismantlement” (CVID) of the country’s nuclear weapons program, with some administration officials even touting the “Libya model” of coercive denuclearization under which the late Libyan leader Moammar Gadhafi agreed in 2003 to roll back his nuclear program in exchange for Tripoli being allowed to return to the international community. Gadhafi was later killed in 2011 by rebels in a U.S.-backed military action during his country’s civil war.

Pyongyang has pointed to this as justification for its own nuclear program, saying it prevents the North Korean regime from suffering the same fate.

The Stanford report — written with Robert Carlin, a former CIA expert on North Korea now at the Center for International Security and Cooperation at the university, and Elliot Serbin, a research assistant to Hecker — said that “insisting on immediate CVID along a ‘Libya model’ to eliminate everything up front and virtually all at once is tantamount to a North Korean surrender scenario.”

Hecker, who has visited North Korean nuclear facilities multiple times and is the only American scientist to see its secretive uranium-enrichment operations, said Tuesday that it is “unimaginable” that Pyongyang would accept immediate, complete denuclearization.

“The phased approach may be acceptable to Kim Jong Un and allow the United States to reduce the greatest risks first and address the manageable risks over time,” he wrote in a tweet. This kind of approach would see “steps that can be phased in over 10 years.”

While North Korea “will surely hedge its bets by retaining parts of the program” initially, Hecker said his study identified risks that would need to be addressed first and lesser risks that would be manageable under a phased program.

According to the study, the most important initial steps for the North to take toward denuclearization are: no nuclear tests, no intermediate- or long-range missile tests, no more production of plutonium and highly enriched uranium, and no export of nuclear weapons, materials or technologies.

Phases would also be split into three, constituting what might be possible during the “halt” stage in the first year, the “roll-back” stage in the second to fifth years, and the “eliminate” stage from the sixth to 10th years.

To make this more palatable to the Trump administration, Pyongyang would have to front-load as much of the denuclearization process as possible, the study said.

It cited the May 24 demolition of the Punggye-ri nuclear test site as one such example.

“We believe several similar moves such as disabling the plutonium-production reactor could be achieved during or before a summit,” the report said, adding that early access to the Nyongbyon centrifuge facility and a halt in operations of uranium chemical processing facilities that support all enrichment activities would be “the most important initial steps” in confronting that challenge.

Beyond the technical issues, the study suggested that a phased approach would also provide an “effective way to build trust and interdependence,” which it said are “required for a viable long-term solution.”

As a part of that, Hecker and his team encouraged Washington to recognize Pyongyang’s desire for civilian nuclear and space programs. The U.S. has traditionally viewed those programs as mere facades for military research.

“Considering the advanced state of North Korea’s nuclear and missile development, we consider the incremental risks posed by the civilian endeavors manageable,” he said.

Ultimately, though, Hecker said that a gradual approach such as the one he suggested hinged on one of Kim’s biggest fears: regime security.

North Korea, he said, will not give up its weapons and its weapons program until its security can be assured.

“Such assurance cannot be achieved simply by a U.S. promise/agreement on paper,” Hecker said. “It will require a substantial period of co-existence and interdependence.”

Still, the study did note that the atmosphere is ripe for such a deal.

“This approach has become much more feasible in the past six months during which the North and South have taken remarkable steps toward reconciliation,” it said. “Together, they can establish a path toward coexistence and interdependence, manage most of the financial burdens, and develop verification protocols that were unimaginable for decades.”

Trump, too, has in recent days appeared to voice some flexibility on the White House’s hard-line CVID stance.

The scale of North Korea’s program, he said ahead of a meeting with South Korean President Moon Jae-in last week, would make it difficult to dismantle it in a single step. “It would certainly be better if it were all in one,” he said. “Does it have to be? I don’t think I want to totally commit myself.”

But as U.S. and North Korean summit negotiators gather in Singapore and at the two Koreas’ border to layout their stances on denuclearization, it remains uncertain whether Trump’s position will continue to remain flexible.