President Donald Trump's top negotiator working on ending Russia's war in Ukraine predicted Trump would have a "successful" phone call with President Vladimir Putin on Monday, saying he believed the conversation "will go a long way towards identifying where we are and how we complete this negotiation."

"I believe that the president is going to have a successful call with -- with Vladimir Putin," U.S. special envoy Steve Witkoff said Sunday on "This Week." "They know each other. The president is determined to get something done here ... if he can't do it, then nobody can."

Delegations from Ukraine and Russia met in Turkey on Thursday for the first direct talks related to ending the war Russia started in February 2022 when it launched a full-scale invasion of its sovereign neighbor. A Ukrainian source told ABC News after those talks that the Russians were making unrealistic demands. A week prior, Vice President JD Vance had said of the Russians, "We think they're asking for too much."

Asked if he shared Vance's assessment, Witkoff told "This Week" co-anchor Jonathan Karl that his "assessment is that this is a very complicated conflict that shouldn't have happened."

"I think in a negotiation like this, people take positions," Witkoff said. "The art here is to narrow ... that, you know, wide berth between the parties. And I think to some extent we've done that. To some extent each party is -- is, you know, staking out their positions. And I think Monday will go a long way towards identifying where we are and how we complete this negotiation."

Trump announced Saturday he will speak with Putin over the phone on Monday at 10 a.m. ET. The president said he planned to speak with Ukrainian President Volodymyr Zelenskyy and "various" NATO allies after his call with Putin.

Here are more highlights from Witkoff's interview:

Says Iranian uranium enrichment 'very clear red line' in nuclear deal talks

Karl: Can you give us -- give us the sense of what is the outline of the deal that President Trump wants to get with Iran?

Witkoff: Well, the president has been very clear, he wants to solve this -- this conflict diplomatically and with dialogue. And he's given -- he's given all the signals. He has directly sent letters to the supreme leader. I have been dispatched to deliver that message as well, and I've delivered it. ...

We have one very, very clear red line, and that is enrichment. We cannot allow even 1 percent of an enrichment capability. We've delivered a proposal to the Iranians that we think addresses some of this without disrespecting them. ... But everything begins for our -- from our standpoint, Jon, with a -- a deal that does not include enrichment. We cannot have that. Because enrichment enables weaponization. And we will not allow a bomb to -- to get here.

But, short of that, there are all kinds of ways for us to achieve our goals in this negotiation. We think that we will be meeting sometime this week in Europe. And we hope that it will lead to some real positivity.

Says many 'initiatives' underway to address humanitarian situation in Gaza

Karl: Are you pushing the Israelis to allow that aid to come in and -- and to hold back on some of the -- the offensive operations that continue to be ongoing in Gaza?

Witkoff: The president, Jon, is a humanitarian. And I think that any -- everyone is concerned about the humanitarian conditions in Gaza. That said, it is a very complicated situation there. Logistically, we are -- I don't think there's any daylight between President Trump's position and Prime Minister Netanyahu's position. The issue now is -- I think the issue now is, how do we logistically get all of those trucks into Gaza? How do we set up the aid stations? There are many things -- initiatives that we are working on to address this. There -- there are going to be mobile kitchens that are going to be sent in there. The flour -- we have trucks with flour waiting at the border. The Israelis have indicated that they're going to begin to allow a lot more of these trucks to get in. But it is complicated. It is logistically complicated.

Dismisses ethics concerns over Qatar gifting 747-8 jet for Air Force One

Witkof: It's a perfectly legal, government to government, Department of Defense, to Department of Defense transaction that happens in the normal course and has been happening in the normal course throughout -- throughout our existence. Governments exchange services. ... They (the Qataris) decided to donate something because of all the wonderful things that we've done for them in the past, and in addition to that, they then, on top of that, announced $1.2 trillion worth of investments into the United States of America, which will create jobs and do all kinds of wonderful things.

So, I don't really -- I'm not sure how anyone would see this as the Qataris looking to gain some sort of advantage. ... I just want to say, this is to the benefit of the United States of America that -- that donation. So, it -- it just has to be seen as that way.