Former U.S. Ambassador to the United Nations Nikki Haley on Tuesday defended President Donald Trump’s July call with the leader of Ukraine, but said that “it’s never a good practice for us to ask a foreign country to investigate an American. It's just not a good practice."

“Having said that, there’s no insistence on that call, there are no demands on that call, it is a conversation between two presidents that’s casual in nature,” Haley said in an interview on "Today" with NBC News' Savannah Guthrie.

According to the White House record of Trump's July 25 call with Ukrainian President Volodymyr Zelenskiy, Trump asked Zelenskiy for a "favor," suggesting the country probe a debunked conspiracy theory about Democrats and the 2016 election and the Biden family. The call is at the center of the House's impeachment inquiry.

Haley contended that it was appropriate for Trump to ask Ukraine to probe "corruption."

"OK, but the corruption mentioned by the president here has to do with Joe Biden and the DNC server," Guthrie said. "Those are the two very specific examples."

"An American should want to know the answer of, 'Did Biden pressure the prosecutor to, you know, to do what he did?' And I think there's a real question there. You can question the president, but you also have to question what Biden did," Haley said.

Trump and his allies have pushed a theory that Biden acted improperly as vice president when he called on Ukraine to fire its top prosecutor, Viktor Shokin, threatening to hold about $1 billion in aid over the country. Shokin had been investigating Burisma Holdings, a natural gas company that Biden's son Hunter sat on the board of. But press accounts say that probe was long dormant by the time Biden pushed for Shokin's ouster, and his removal as prosecutor was the aim of a number of countries and international bodies. Shokin was accused of ignoring corruption in Ukraine, not pursuing it.

As Rep. Jim Himes, D-Conn., put it in an interview with NBC's "Meet the Press" on Sunday, what Biden had asked for was for the benefit of U.S. foreign policy with the backing of the international community. Democrats have alleged Trump was seeking investigations that would prove politically beneficial to him by directing a pressure campaign that included freezing millions of dollars in Congress-approved military aid that Ukraine depended on to counter Russian aggression.

In the interview, Guthrie also pressed Haley on Trump's fitness for office and her claims that top officials sought to undermine the president.

Haley addressed a portion of her new book, "With All Due Respect," where she detailed a meeting she had with Trump after that July 2018 press conference, in which the president appeared to side with Putin over U.S. intelligence agencies' determination that Russia interfered in the 2016 election.

In the meeting Haley recalled, she told Trump he did not do well alongside Putin, a point that surprised Trump because, in Haley's retelling, she was the only person to tell him that.

Guthrie pressed Haley on that claim.

"Really? You were the only person?" Guthrie asked. "I mean that news conference was globally condemned because of that moment, but you were the only person in the administration who said, Hey, that didn’t look so great?"

"That’s what he told me," Haley responded. "I mean when I said I wanted to meet with him, and I go through that in the book, when I said I wanted to meet with him, and I said, 'Look, this sounded soft.' And he said, 'Really?' John Kelly was in the room with me when I had this meeting, and he looked at John and he said, 'All of you guys said I did great.'"

Guthrie pointed to separate claims from Haley's book, in which she said Kelly, then the White House chief of staff, and then-Secretary of State Rex Tillerson, had sought to undermine Trump's decisions in order to save the country, and asked how she could explain Kelly's offering Trump such a positive assessment of his meeting with Putin in light of those assertions.

"You ask him those questions," Haley said, expressing support for Trump's policies toward Russia. "But the issue was that on that topic, no one had said anything to him, and I thought it was hugely important, and you’ll see later, he comes out, and he comes out much stronger on Russia."

Haley also told Guthrie that she did tell Trump of what she described as Kelly and Tillerson's insubordination — an effort to undermine the president that Haley claims they recruited her to be a part of.

Asked if and when such a conversation between Haley and Trump took place, as well as what the president thought of Haley's assertions, White House press secretary Stephanie Grisham told NBC News, "I believe that’s something you’d need to ask Nikki."

Speaking with NBC's Craig Melvin later Tuesday on "Today," Haley said she would not "get into the president's response" to her allegations.

"So when this first happened, the first person I went to was the National Security Adviser, H.R. McMaster," Haley added. "And he knew too. I mean he was struggling to get things done, and they were stalling with him as well, and his job was to implement the president’s policy. And so, he was as frustrated as I was, but he was the first one I went to. And then, obviously, you know, telling the president, you want to protect what his role is."

"And this was just on policy discussions," she said. "This was on the Iran deal. This was on Paris Climate. This wasn’t on him being, you know, out there being not able to control himself."

Melvin asked if she was surprised Trump kept both men on after she told him of her claims, to which she said, "Well, they're gone."

"So, I told him in due time, and he figured it out on his own, and the rest he was able to figure out," she said of Trump.

In response to Haley's claims, Kelly told The Washington Post Sunday that if giving Trump "the best and most open, legal and ethical staffing advice from across the [government] so he could make an informed decision is 'working against Trump,' then guilty as charged."

Tillerson, meanwhile, told NBC News that "at no time did I, nor to my direct knowledge did anyone else serving along with me, take any actions to undermine the president."

"Ambassador Haley was rarely a participant in my many meetings, and is not in a position to know what I may not have said to the president," he added.

Commenting on Tillerson's statement, Haley said, "that's Rex."

"I was in every Cabinet meeting, I was in every National Security Council meeting, I was in multiple Oval Office meetings, and what’s fascinating, Craig, is Rex and Kelly have never denied that the meeting happened," Haley said. "They’ve never denied that they did sit me down, that they did say those words, and they know it."

Guthrie concluded her interview by asking Haley whether Trump is honest and fit for office. Haley said she never doubted Trump's mental acuity.

"In every instance that I dealt with him, he was truthful, he listened, and he was great to work with," she said.

In the later interview with Melvin, Haley was asked if Trump should release his tax returns, which he has shielded from the public, breaking with four decades of tradition.

"I always err on the side of transparency, but the American people didn't care what his tax returns said," she said.

Asked if she suggested to Trump that he release them, Haley said, "No, because he got elected."

"He got elected without releasing them," she said.