U.S. Secretary of State Mike Pompeo said “real progress” had been made in negotiations Thursday with North Korean leader Kim Jong Un’s top deputy in New York toward holding a potential summit between Kim and President Donald Trump.

“We’ve made real progress in the last 72 hours in setting the conditions,” Pompeo said at a news conference after the meeting, adding “there remains a great deal of work to do.”

Pompeo said he believes North Korea is contemplating a new strategic path forward, but he cautioned that the country would “have to choose a path that is fundamentally different” to achieve security — meaning complete, verifiable, irreversible denuclearization.

“Our two countries face a pivotal moment in our relationship in which it could be nothing short of tragic to let this opportunity go to waste,” Pompeo cautioned.

He said Kim Yong Chol, the highest-ranking North Korean official to visit the United States in 18 years, is planning to travel to Washington on Friday to deliver a personal letter from Kim Jong Un.

Pompeo and Kim Yong Chol met at a U.S. government-leased apartment near the United Nations. They talked over a steak dinner Wednesday and chocolate croissants Thursday morning.

Photos from the encounter show a smiling Secretary Pompeo shaking hands and clinking glasses with the North Korean. In another picture, he is gesturing out a window toward the expansive view of the New York skyline with his smiling guest looking on.

The two men have met twice before in Pyongyang.

Earlier, President Trump said talks between Pompeo and Kim Yong Chol had been going well and he hoped to hold a summit with Kim Jong Un in Singapore on the originally scheduled date of June 12. Trump said he looks forward to seeing what is in the letter the North Korean delegation is bringing to Washington.

Trump later told Reuters news agency it may take more than one meeting with Kim Jong Un to seal a denuclearization deal with North Korea and that he would like Pyongyang to dismantle its nuclear weapons program as quickly as possible under any agreement.

Discussions between the two sides also are taking place in Panmunjom and Singapore.

Meanwhile, North Korean leader Kim Jong Un and Russian Foreign Minister Sergei Lavrov met in Pyongyang Thursday, according to North Korea’s official state news agency. KCNA said Kim told Lavrov that he hopes North Korea’s issues with the United States — including the state of U.S.-Korean relations and the denuclearization of the Korean peninsula — can be “solved on a stage-by-stage basis” through “effective and constructive dialogue and negotiation.”

But the White House says talks of the “total denuclearization of the peninsula” do not extend to U.S. weapons systems — a defense umbrella covering South Korea that includes nuclear-armed submarines and strategic bombers capable of carrying nuclear bombs not based on the peninsula.

North Korea is estimated to have more than a dozen nuclear weapons.

An advance team, led by Deputy Chief of Staff Joe Hagin, has held meetings with the North Korean team in Singapore this week.