President Donald Trump's top negotiator working on 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. The president said he planned to speak with Ukrainian President Volodymyr Zelenskyy and "various" NATO allies after his call with Putin.

This is a developing story. Please check back for updates.