WASHINGTON—The U.S. will be unable to continue providing more weapons and equipment to Ukraine if Congress doesn’t approve additional funding by the end of the year, the White House said, trying to prod lawmakers to quickly make progress in complicated negotiations on Capitol Hill.

“Cutting off the flow of U.S. weapons and equipment will kneecap Ukraine on the battlefield,” Shalanda Young, the director of the White House Office of Management and Budget, wrote in a letter to House Speaker Mike Johnson (R., La.). “There is no magical pot of funding available to meet this moment. We are out of money—and nearly out of time,” Young wrote.