"There is a very real risk that the Ukrainians could lose on the battlefield by the end of 2024, or at least put [Russian President Vladimir Putin] in a position where he could essentially dictate the terms of a political settlement," Burns said during an event at the George W. Bush Presidential Center.
However, further military assistance from the US Congress could enable Ukraine to hold out through 2024, Burns said.
The US House of Representatives is considering a bill this week with $60.84 billion in Ukraine-related funding, as part of a broader set of legislation to aid Ukraine, Israel, and Taiwan.
Earlier on Thursday, US House Foreign Affairs Committee Chairman Michael McCaul said that Ukraine has between two weeks and one month before Russia "takes over" the country.