The Trump administration has taken a harder-line approach to denuclearizing North Korea since the summit in Vietnam last month, raising doubts about whether the two sides will reach a deal on the centerpiece of President Trump’s foreign policy.

In remarks Monday, a top U.S. envoy said the United States would not lift sanctions on North Korea until it completely dismantles its nuclear and ballistic missiles. The United States is also seeking an end to Pyongyang’s chemical and biological weapons, he said.

“We are not going to do de­nuclearization incrementally. The president has been clear on that,” Stephen Biegun, the special U.S. envoy for North Korea, said at a forum hosted by the Carnegie Endowment for International Peace in Washington.

Biegun added that there was “complete unity” inside the Trump administration on that approach.

North Korea has long insisted that any steps it takes to denuclearize must be met with corresponding measures from the United States, including relief from economic sanctions.

The Trump administration’s apparent rejection of that approach has left analysts baffled over where the two sides might find room to negotiate an eventual deal.

“If we’re going to stay firm on the maximalist position, it’s hard to see where we go from here because there’s no way Kim is going to accept this,” said Vipin Narang, a North Korea expert at the Massachusetts Institute of Technology.

In advance of Trump’s meeting with North Korean leader Kim Jong Un in Vietnam, U.S. negotiators considered a more modest deal that would trade some sanctions relief in exchange for the dismantling of the Yongbyon nuclear complex, said diplomats familiar with the negotiations.

The Yongbyon complex is a critical part of North Korea’s nuclear program, but the country is believed to maintain missiles, warheads and enrichment facilities in other areas as well.

Although Biegun appeared to be in favor of a more incremental approach before the summit, Trump’s national security adviser, John Bolton, has long opposed that strategy and has privately expressed his view that Biegun was offering too much to the North Koreans.

Confronted with North Korea’s insistence on major sanctions relief at the summit, the United States made a counterproposal demanding full sanctions relief for full denuclearization, said the diplomats, who spoke on the condition of anonymity to discuss sensitive negotiations.

North Korea’s refusal and the emergence of satellite imagery that appears to show activity at North Korean launch and assembly sites have fueled uncertainty about the future of the talks.

Biegun said he could not explain the construction activity at the sites. Although he said the United States takes it “very seriously,” he cautioned against drawing conclusions.

In his remarks, Biegun insisted that “diplomacy is still very much alive” with North Korea, but the absence of any scheduled meetings between Washington and Pyongyang, and the initiation of suspicious construction activity at the Sohae launch site has raised concerns that the positive atmosphere between Trump and Kim could quickly unravel.

“There is a serious risk of drift, and the North Koreans do appear to be sending a signal with the operations at Sohae that, ‘Yes, we can reverse our nuclear and ballistic missile-testing halt,’ ” said Daryl Kimball, executive director of the Arms Control Association. “It’s a sobering reminder that without an agreement that verifies the dismantlement of North Korea’s program, we can easily go back to a period of ‘fire and fury’ rhetoric.”