Washington (CNN) Republican Sen. James Lankford said Sunday that he would not echo President Donald Trump by calling North Korean leader Kim Jong Un honorable, and said Trump would be better served to return to his nickname for the dictator.

"I would never use the word honorable to describe Kim Jong Un," Lankford, of Oklahoma, said on CNN's "State of the Union." "I think he's better to be able to just call him 'rocket man' and to be able to stick with that than honorable just because he is a ruthless dictator that does public executions of anyone who disagrees."

Lankford's comment was one of several from lawmakers and top administration officials on North Korea this Sunday as negotiations for a meeting between Trump and Kim have been underway.

Trump on Tuesday offered praise for Kim, saying ahead of a potential meeting that the North Korean leader "has really been very open and I think very honorable based on what we are seeing."

The comment, which he did not explain, came in contrast to Trump's previous mocking of Kim, labeling him "rocket man" on Twitter and in public remarks.

Bolton: Trump 'eager' for meeting soon

Trump said Saturday night that talks with Kim could happen in "three or four weeks," and newly minted White House national security adviser John Bolton said Sunday that Trump wants to have the meeting soon.

"I think the President is eager to do it as soon as possible, but we still need to work out the precise parameters," Bolton said on "Fox News Sunday."

Bolton said they had not yet agreed on a place for the meeting. He also said he did not see how it would be possible for the US to accept North Korea continuing to be a nuclear power.

Denuclearization would mean North Korea agreeing to give up its nuclear program before the US makes concessions, Bolton said, adding that the administration has in mind something similar to the agreement the US and United Kingdom made with Libya during the rule of now-deceased dictator Moammar Gadhafi.

"We have very much in mind the Libya model from 2003, 2004," Bolton said. "There are obviously differences. The Libyan program was much smaller. But that was basically the agreement that we made."

In an interview on CBS's "Face the Nation," Bolton said that in addition to the Libya case, they were looking at previous actions by North Korea, including a 1992 nuclear agreement between the Pyongyang and South Korea.

Bolton also said that outside of the nuclear program, there was a range of other concerns to address, including other weapons programs and people held captive by North Korea.

"We've got other issues to discuss as well," Bolton said. "Their ballistic missile programs, their biological and chemical weapons programs, their keeping of American hostages, the abduction of innocent Japanese and South Korean citizens over the years. So there's a lot to talk about."

Secretary of State Mike Pompeo said in an interview with ABC that aired Sunday that when he met with Kim several weeks ago, they talked about denuclearization and three US prisoners held in North Korea.

"I talked about getting the release of the American detainees," Pompeo said. "And then we talked a great deal about what it might look like, what this complete, verifiable, irreversible mechanism might look like. And so, when the two leaders, the only people that can make those decisions, will be in a room together, they can set the course."

Assigning credit

In Sunday's interview, Lankford backed the international effort to pressure North Korea over its nuclear program and offered Trump a share of the credit for North Korea's overtures towards diplomacy.

Trump said at a rally Saturday evening that South Korean President Moon Jae-In "gives us all the credit" for the apparent progress with North Korea.

When asked about Trump's boasting on the situation, Lankford said, "He's part of the journey, and there's no question on that."

"President Trump has put in a set of sanctions that has actually brought North Korea to the table," Lankford added.

The senator, who sits on the Senate Intelligence Committee, went on to credit South Korea, Japan and China for their roles in pressuring North Korea.

"This international effort is bringing North Korea actually to the table, where they should be," Lankford said.

Ohio Gov. John Kasich said in a separate interview on the same program that he credited the international sanctions effort and pressure on North Korea from Trump for the changing situation.

"There are many factors involved," Kasich, a Republican, said. "And part of it is the pressure that Donald Trump put on the North Koreans. But it's also the rest of the world that joined in, including the Chinese, to say we're going to sanction them."