Mike Pompeo, the current CIA director, told members of the Senate Foreign Relations considering his nomination Thursday that "no one is under any illusions that we will reach a comprehensive agreement through the president's meeting" with Kim Jong Un in May or early June.
The summit could lay out conditions that would allow the leaders to decide whether a deal was possible, Pompeo said. That could "set us down the course to achieve a diplomatic outcome that America and the world so desperately need," he said.
At the White House, Trump's new national security adviser held talks with Japanese and South Korean officials. John Bolton, a major player in preparing Trump for his historic get-together with Kim, has in the past advocated military force against North Korea and other US adversaries.
Pompeo described a goal of getting the North Korean leadership to agree to "step away from its efforts to hold America at risk with nuclear weapons."
He tried to make the case that he could transition from spy chief to diplomat, asserted he was not a "hawk" and said he had never supported efforts to remove North Korea's rulers. Pompeo agreed with Sen. Ed Markey, D-Mass., that it would be "catastrophic" if the US initiated an attack against North Korea, but he didn't rule out the possibility of military action.
Bolton met with Chung Eui-yong, the South Korean official who last month conveyed Kim's offer to Trump to talk about "denuclearization" and halt nuclear and missile tests. Chung was then given the unexpected duty of announcing to journalists in the White House driveway Trump's surprise agreement to hold a meeting.
South Korea's president, Moon Jae-in, is due to hold his own summit with Kim on April 27.
Bolton also held talks with Japan's Shotaro Yachi, whose prime minister, Shinzo Abe, will meet Trump at his Florida resort on Tuesday.
Abe has fostered personal ties with Trump and has been a strong supporter of the US-led campaign of sanctions against North Korea to compel it to negotiate on its nuclear program after a long hiatus in talks.
During his second trip to Mar-a-Lago, Abe will be seeking reassurance that Japan's interests won't be overlooked at the Trump-Kim summit.
The Japanese leader has said he's worried that North Korean security threats to Japan, including short- and medium-range missiles, may not be discussed, and that Trump will focus on intercontinental ballistic missiles that threaten America.
The differing perspectives of the allies, which both host US forces but have sharp historic differences, could complicate Washington's management of policy on North Korea.
A spokesman at the National Security Council said Chung and Yachi are the first national security advisers to meet with Bolton since he took office this week.
Trump on Thursday praised the role of China, currently in a standoff on trade tariffs with the US, in pressuring North Korea. He predicted his meeting with Kim would be "terrific."
"I think we're going with a lot of respect, and we'll see what happens. So we've come a long way. But China has really helped us at the border, and we appreciate it," Trump said at a meeting with governors and senators on trade and agriculture at the White House.(AP)