The CIA chief warned Thursday that Ukraine could lose the war to Russia by the end of this year unless the United States sends more military aid.

The grim warning by Bill Burns came as the US House of Representatives is scheduled to vote Saturday on a $61 billion aid package for Ukraine that has been held up for months by Republican lawmakers allied with Donald Trump as he seeks reelection.

The administration of President Joe Biden has been warning as it lobbied to send more ammunition and other materiel to Ukraine that the pro-Western US ally could lose territory to Russia's invasion forces unless it got more American help.

But on Thursday, in a speech at the George W. Bush Center, Burns said the stakes are now much higher.

"With the boost that would come from military assistance, both practically and psychologically, I think the Ukrainians are entirely capable of holding their own through 2024," Burns said.

"Without supplemental assistance, the picture is a lot more dire," he added.

"There is a very real risk that the Ukrainians could lose on the battlefield by the end of 2024, or at least put Putin in a position where he could dictate the terms of a political settlement," he said, referring to Russia's president.

Burns did not specify what he meant by "lose" the war.

He also tried to illustrate how hard up the Ukrainian forces are for fresh ammunition.

Burns said two brigades - 2,000-plus-soldier units - had "15 artillery rounds per day" and "a grand total of 42 mortar rounds," respectively.

The House vote Saturday is one of a series of votes on separate aid packages for Ukraine, Israel and Taiwan.

Ukrainian President Volodymyr Zelensky has also issued dire warnings that his country will lose the war unless it gets more military assistance.