New Delhi, Aug. 18 -- Russian leader Vladimir Putin agreed at his summit with President Donald Trump that the United States and its European allies could offer Ukraine a security guarantee resembling NATO's collective defense mandate as part of an eventual deal to end the war, a U.S. official said Sunday.

Trump envoy Steve Witkoff, who took part in the talks Friday at a military base in Alaska, said it "was the first time we had ever heard the Russians agree to that" and called it "game-changing."

"We were able to win the following concession: that the United States could offer Article 5-like protection, which is one of the real reasons why Ukraine wants to be in NATO," Witkoff told CNN's "State of the Union."

Witkoff offered few detai...