The U.S. government could run out of resources to meet the nation’s obligations as soon as Dec. 15, Treasury Secretary Janet Yellen said Tuesday, reviving questions about how Congress will resolve a standoff about raising or suspending the federal borrowing limit.

Ms. Yellen provided the new estimate of when the federal government might no longer be able to pay all of its bills in a letter to Congressional leaders. She had previously said that a debt-limit increase passed by Congress in October provided confidence that the federal government would be able to pay its bills at least through Dec. 3.